Chapter 6 Data visualization

6.1 Plotting with ggplot2

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ggplot2 is a plotting package that makes it simple to create complex plots from data in a data frame. It provides a more programmatic interface for specifying what variables to plot, how they are displayed, and general visual properties. The theoretical foundation that supports the ggplot2 is the Grammar of Graphics (Wilkinson (⊕2005Wilkinson, Leland. 2005. The Grammar of Graphics (Statistics and Computing). Berlin, Heidelberg: Springer-Verlag.)). Using this approach, we only need minimal changes if the underlying data change or if we decide to change from a bar plot to a scatterplot. This helps in creating publication quality plots with minimal amounts of adjustments and tweaking.

There is a book about ggplot2 (Wickham (⊕2016Wickham, Hadley. 2016. ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag New York. http://ggplot2.org.)) that provides a good overview, but it is outdated. The 3rd edition is in preparation and will be freely available online. The ggplot2 webpage (https://ggplot2.tidyverse.org) provides ample documentation.

ggplot2 functions like data in the ‘long’ format, i.e., a column for every dimension, and a row for every observation. Well-structured data will save you lots of time when making figures with ggplot2.

ggplot graphics are built step by step by adding new elements. Adding layers in this fashion allows for extensive flexibility and customization of plots.

The idea behind the Grammar of Graphics it is that you can build every graph from the same 3 components: (1) a data set, (2) a coordinate system, and (3) geoms — i.e. visual marks that represent data points ¹⁰.

To build a ggplot, we will use the following basic template that can be used for different types of plots:

ggplot(data = <DATA>, mapping = aes(<MAPPINGS>)) +  <GEOM_FUNCTION>()

• use the ggplot() function and bind the plot to a specific data frame using the data argument

ggplot(data = rna)

• define a mapping (using the aesthetic (aes) function), by selecting the variables to be plotted and specifying how to present them in the graph, e.g. as x/y positions or characteristics such as size, shape, color, etc.

ggplot(data = rna, mapping = aes(x = expression))

• add ‘geoms’ – geometries, or graphical representations of the data in the plot (points, lines, bars). ggplot2 offers many different geoms; we will use some common ones today, including:

  * `geom_point()` for scatter plots, dot plots, etc.
  * `geom_histogram()` for histograms
  * `geom_boxplot()` for, well, boxplots!
  * `geom_line()` for trend lines, time series, etc.

To add a geom(etry) to the plot use the + operator. Let’s use geom_histogram() first:

ggplot(data = rna, mapping = aes(x = expression)) +
  geom_histogram()

## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

The + in the ggplot2 package is particularly useful because it allows you to modify existing ggplot objects. This means you can easily set up plot templates and conveniently explore different types of plots, so the above plot can also be generated with code like this:

# Assign plot to a variable
rna_plot <- ggplot(data = rna,
                   mapping = aes(x = expression))

# Draw the plot
rna_plot + geom_histogram()

## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

# change bins
ggplot(rna, aes(x = expression)) +
    geom_histogram(bins = 15)

# change binwidth
ggplot(rna, aes(x = expression)) +
    geom_histogram(binwidth = 2000)

We can observe here that the data are skewed to the right. We can apply log2 transformation to have a more symmetric distribution. Note that we add here a small constant value (+1) to avoid having -Inf values returned for expression values equal to 0.

rna <- rna %>%
  mutate(expression_log = log2(expression + 1))

If we now draw the histogram of the log2-transformed expressions, the distribution is indeed closer to a normal distribution.

ggplot(rna, aes(x = expression_log)) + geom_histogram()

## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

From now on we will work on the log-transformed expression values.

ggplot(data = rna,mapping = aes(x = expression))+
  geom_histogram() +
  scale_x_log10()

## Warning in scale_x_log10(): log-10 transformation introduced infinite values.

## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

## Warning: Removed 507 rows containing non-finite outside the scale range
## (`stat_bin()`).

Notes

• Anything you put in the ggplot() function can be seen by any geom layers that you add (i.e., these are global plot settings). This includes the x- and y-axis mapping you set up in aes().
• You can also specify mappings for a given geom independently of the mappings defined globally in the ggplot() function.
• The + sign used to add new layers must be placed at the end of the line containing the previous layer. If, instead, the + sign is added at the beginning of the line containing the new layer, ggplot2 will not add the new layer and will return an error message.

# This is the correct syntax for adding layers
rna_plot +
  geom_histogram()

# This will not add the new layer and will return an error message
rna_plot
  + geom_histogram()

6.2 Building your plots iteratively

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We will now draw a scatter plot with two continuous variables and the geom_point() function. This graph will represent the log2 fold changes of expression values comparing time 8 versus time 0 and time 4 versus time 0. To this end, we first need to compute the means of the log-transformed expression values by gene and time then the log fold changes by subtracting the mean log expressions between time 8 and time 0 and between time 4 and time 0. Note that we also include here the gene biotype that we will use later on to represent the genes. We will save the fold changes in a new data frame called rna_fc.

rna_fc <- rna %>% select(gene, time,
                         gene_biotype, expression_log) %>%
  group_by(gene, time, gene_biotype) %>%
  summarize(mean_exp = mean(expression_log)) %>%
  pivot_wider(names_from = time,
              values_from = mean_exp) %>%
  mutate(time_8_vs_0 = `8` - `0`, time_4_vs_0 = `4` - `0`)

## `summarise()` has grouped output by 'gene', 'time'. You can override using the `.groups`
## argument.

We can then build a ggplot with the newly created dataset rna_fc. Building plots with ggplot2 is typically an iterative process. We start by defining the dataset we’ll use, lay out the axes, and choose a geom:

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0)) +
    geom_point()

Then, we start modifying this plot to extract more information from it. For instance, we can add transparency (alpha) to avoid overplotting:

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0)) +
    geom_point(alpha = 0.3)

We can also add colors for all the points:

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0)) +
    geom_point(alpha = 0.3, color = "blue")

Or to color each gene in the plot differently, you could use a vector as an input to the argument color. ggplot2 will provide a different color corresponding to different values in the vector. Here is an example where we color with gene_biotype:

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0)) +
    geom_point(alpha = 0.3,
               aes(color = gene_biotype))

## Warning in vp$just: partial match of 'just' to 'justification'

We can also specify the colors directly inside the mapping provided in the ggplot() function. This will be seen by any geom layers and the mapping will be determined by the x- and y-axis set up in aes().

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0,
                     color = gene_biotype)) +
    geom_point(alpha = 0.3)

## Warning in vp$just: partial match of 'just' to 'justification'

Finally, we could also add a diagonal line with the geom_abline() function:

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0,
                     color = gene_biotype)) +
    geom_point(alpha = 0.3) +
    geom_abline(intercept = 0)

## Warning in vp$just: partial match of 'just' to 'justification'

Notice that we can change the geom layer and colors will be still determined by gene_biotype

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0,
                     color = gene_biotype)) +
    geom_jitter(alpha = 0.3) +
    geom_abline(intercept = 0)

## Warning in vp$just: partial match of 'just' to 'justification'

install.packages("hexbin")

library("hexbin")

ggplot(data = rna_fc,
       mapping = aes(x = time_4_vs_0,
                     y = time_8_vs_0)) +
    geom_hex() +
    geom_abline(intercept = 0)

## Warning in vp$just: partial match of 'just' to 'justification'

ggplot(data = rna,
       mapping = aes(y = expression_log,
                     x = sample)) +
    geom_point(aes(color = time))

## Warning in vp$just: partial match of 'just' to 'justification'

6.3 Boxplot

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We can use boxplots to visualize the distribution of gene expressions within each sample:

ggplot(data = rna,
       mapping = aes(y = expression_log, x = sample)) +
    geom_boxplot()

By adding points to boxplot, we can have a better idea of the number of measurements and of their distribution:

ggplot(data = rna,
       mapping = aes(y = expression_log,
                     x = sample)) +
    geom_jitter(alpha = 0.2, color = "tomato") +
    geom_boxplot(alpha = 0)

ggplot(data = rna,
         mapping = aes(y = expression_log, x = sample)) +
  geom_boxplot(alpha = 0) +
  geom_jitter(alpha = 0.2, color = "tomato")

You may notice that the values on the x-axis are still not properly readable. Let’s change the orientation of the labels and adjust them vertically and horizontally so they don’t overlap. You can use a 90-degree angle, or experiment to find the appropriate angle for diagonally oriented labels:

ggplot(data = rna,
       mapping = aes(y = expression_log,
                     x = sample)) +
    geom_jitter(alpha = 0.2, color = "tomato") +
    geom_boxplot(alpha = 0) +
    theme(axis.text.x = element_text(angle = 90, hjust = 0.5, vjust = 0.5))

# time as integer
ggplot(data = rna,
         mapping = aes(y = expression_log,
                       x = sample)) +
  geom_jitter(alpha = 0.2, aes(color = time)) +
  geom_boxplot(alpha = 0) +
  theme(axis.text.x = element_text(angle = 90, hjust = 0.5, vjust = 0.5))

## Warning in vp$just: partial match of 'just' to 'justification'

# time as factor
ggplot(data = rna,
         mapping = aes(y = expression_log,
                       x = sample)) +
  geom_jitter(alpha = 0.2, aes(color = as.factor(time))) +
  geom_boxplot(alpha = 0) +
  theme(axis.text.x = element_text(angle = 90, hjust = 0.5, vjust = 0.5))

## Warning in vp$just: partial match of 'just' to 'justification'

ggplot(data = rna,
         mapping = aes(y = expression_log,
                       x = sample)) +
    geom_violin(aes(fill = as.factor(time))) +
    theme(axis.text.x = element_text(angle = 90,  hjust = 0.5, vjust = 0.5))

## Warning in vp$just: partial match of 'just' to 'justification'

ggplot(data = rna,
       mapping = aes(y = expression_log,
                     x = sample)) +
    geom_violin(aes(fill = sex)) +
    theme(axis.text.x = element_text(angle = 90, hjust = 0.5, vjust = 0.5))

## Warning in vp$just: partial match of 'just' to 'justification'

6.4 Line plots

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Let’s calculate the mean expression per duration of the infection for the 10 genes having the highest log fold changes comparing time 8 versus time 0.

First, we need to select the genes and create a subset of rna called sub_rna containing the 10 selected genes, then we need to group the data and calculate the mean gene expression within each group:

genes_selected <- rna_fc %>%
    arrange(desc(time_8_vs_0)) %>%
    head(10) %>%
    pull(gene)

sub_rna <- rna %>%
    filter(gene %in% genes_selected)

We can now calculate the mean expressions over time for these selected genes:

mean_exp_by_time <- sub_rna %>%
    group_by(gene,time) %>%
    summarize(mean_exp = mean(expression_log))

## `summarise()` has grouped output by 'gene'. You can override using the
## `.groups` argument.

We can build the line plot with duration of the infection on the x-axis and the mean expression on the y-axis:

ggplot(data = mean_exp_by_time,
       mapping = aes(x = time, y = mean_exp)) +
    geom_line()

Unfortunately, this does not work because we plotted data for all the genes together. We need to tell ggplot to draw a line for each gene by modifying the aesthetic function to include group = gene:

ggplot(data = mean_exp_by_time,
       mapping = aes(x = time,y = mean_exp,
                     group = gene)) +
    geom_line()

We will be able to distinguish genes in the plot if we add colors (using color also automatically groups the data):

ggplot(data = mean_exp_by_time,
       mapping = aes(x = time, y = mean_exp,
                     color = gene)) +
  geom_line()

## Warning in vp$just: partial match of 'just' to 'justification'

6.5 Faceting

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ggplot2 has a special technique called faceting that allows the user to split one plot into multiple (sub) plots based on a factor included in the dataset. These different subplots inherit the same properties (axes limits, ticks, …) to facilitate their direct comparison. We will use it to make a line plot across time for each gene:

ggplot(data = mean_exp_by_time,
       mapping = aes(x = time, y = mean_exp)) +
    geom_line() +
    facet_wrap(~ gene)

Here both x- and y-axis have the same scale for all the sub plots. You can change this default behavior by modifying scales in order to allow a free scale for the y-axis:

ggplot(data = mean_exp_by_time,
       mapping = aes(x = time,
                     y = mean_exp)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y")

Now we would like to split the line in each plot by the sex of the mice. To do that we need to calculate the mean expression in the data frame grouped by gene, time, and sex:

mean_exp_by_time_sex <- sub_rna %>%
  group_by(gene, time, sex) %>%
  summarize(mean_exp = mean(expression_log))

## `summarise()` has grouped output by 'gene', 'time'. You can override using the `.groups`
## argument.

We can now make the faceted plot by splitting further by sex using color (within a single plot):

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y")

## Warning in vp$just: partial match of 'just' to 'justification'

Usually plots with white background look more readable when printed. We can set the background to white using the function theme_bw(). Additionally, we can remove the grid:

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y") +
    theme_bw() +
    theme(panel.grid = element_blank())

## Warning in vp$just: partial match of 'just' to 'justification'

mean_exp_by_chromosome <- rna %>%
  group_by(chromosome_name, time) %>%
  summarize(mean_exp = mean(expression_log))

## `summarise()` has grouped output by 'chromosome_name'. You can override using the `.groups`
## argument.

ggplot(data = mean_exp_by_chromosome,
       mapping = aes(x = time,
                     y = mean_exp)) +
    geom_line() +
    facet_wrap(~ chromosome_name, scales = "free_y")

The facet_wrap geometry extracts plots into an arbitrary number of dimensions to allow them to cleanly fit on one page. On the other hand, the facet_grid geometry allows you to explicitly specify how you want your plots to be arranged via formula notation (rows ~ columns; a . can be used as a placeholder that indicates only one row or column).

Let’s modify the previous plot to compare how the mean gene expression of males and females has changed through time:

# One column, facet by rows
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = gene)) +
    geom_line() +
    facet_grid(sex ~ .)

## Warning in vp$just: partial match of 'just' to 'justification'

# One row, facet by column
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = gene)) +
    geom_line() +
    facet_grid(. ~ sex)

## Warning in vp$just: partial match of 'just' to 'justification'

6.6 ggplot2 themes

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In addition to theme_bw(), which changes the plot background to white, ggplot2 comes with several other themes which can be useful to quickly change the look of your visualization. The complete list of themes is available at http://docs.ggplot2.org/current/ggtheme.html. theme_minimal() and theme_light() are popular, and theme_void() can be useful as a starting point to create a new hand-crafted theme.

The ggthemes package provides a wide variety of options (including an Excel 2003 theme). The ggplot2 extensions website provides a list of packages that extend the capabilities of ggplot2, including additional themes.

6.7 Customisation

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Let’s come back to the faceted plot of mean expression by time and gene, colored by sex.

Take a look at the ggplot2 cheat sheet and think of ways you could improve the plot.

Now, we can change names of axes to something more informative than ‘time’ and ‘mean_exp’, and add a title to the figure:

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y") +
    theme_bw() +
    theme(panel.grid = element_blank()) +
    labs(title = "Mean gene expression by duration of the infection",
         x = "Duration of the infection (in days)",
         y = "Mean gene expression")

## Warning in vp$just: partial match of 'just' to 'justification'

The axes have more informative names, but their readability can be improved by increasing the font size:

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y") +
    theme_bw() +
    theme(panel.grid = element_blank()) +
    labs(title = "Mean gene expression by duration of the infection",
       x = "Duration of the infection (in days)",
       y = "Mean gene expression")  +
    theme(text = element_text(size = 16))

## Warning in vp$just: partial match of 'just' to 'justification'

Note that it is also possible to change the fonts of your plots. If you are on Windows, you may have to install the extrafont package, and follow the instructions included in the README for this package.

We can further customize the color of x- and y-axis text, the color of the grid, etc. We can also for example move the legend to the top by setting legend.position to "top".

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time,
                     y = mean_exp,
                     color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y") +
    theme_bw() +
    theme(panel.grid = element_blank()) +
    labs(title = "Mean gene expression by duration of the infection",
       x = "Duration of the infection (in days)",
       y = "Mean gene expression")  +
    theme(text = element_text(size = 16),
          axis.text.x = element_text(colour = "royalblue4", size = 12),
          axis.text.y = element_text(colour = "royalblue4", size = 12),
          panel.grid = element_line(colour="lightsteelblue1"),
          legend.position = "top")

## Warning in vp$just: partial match of 'just' to 'justification'

If you like the changes you created better than the default theme, you can save them as an object to be able to easily apply them to other plots you may create. Here is an example with the histogram we have previously created.

blue_theme <-
    theme(axis.text.x = element_text(colour = "royalblue4",
                                     size = 12),
          axis.text.y = element_text(colour = "royalblue4",
                                     size = 12),
          text = element_text(size = 16),
          panel.grid = element_line(colour="lightsteelblue1"))

ggplot(rna, aes(x = expression_log)) +
    geom_histogram(bins = 20) +
    blue_theme

ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time, y = mean_exp, color = sex)) +
  geom_line() +
  facet_wrap(~ gene, scales = "free_y") +
  theme_bw() +
  theme(panel.grid = element_blank())

## Warning in vp$just: partial match of 'just' to 'justification'

# change the thickness of the lines
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time, y = mean_exp, color = sex)) +
  geom_line(size=1.5) +
  facet_wrap(~ gene, scales = "free_y") +
  theme_bw() +
  theme(panel.grid = element_blank())

## Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
## ℹ Please use `linewidth` instead.
## This warning is displayed once every 8 hours.
## Call `lifecycle::last_lifecycle_warnings()` to see where this warning was generated.

## Warning in vp$just: partial match of 'just' to 'justification'

# change the name of the legend and the labels
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time, y = mean_exp, color = sex)) +
  geom_line() +
  facet_wrap(~ gene, scales = "free_y") +
  theme_bw() +
  theme(panel.grid = element_blank()) +
  scale_color_discrete(name = "Gender", labels = c("F", "M"))

## Warning in vp$just: partial match of 'just' to 'justification'

# using a different color palette
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time, y = mean_exp, color = sex)) +
  geom_line() +
  facet_wrap(~ gene, scales = "free_y") +
  theme_bw() +
  theme(panel.grid = element_blank()) +
  scale_color_brewer(name = "Gender", labels = c("F", "M"), palette = "Dark2")

## Warning in vp$just: partial match of 'just' to 'justification'

# manually specifying the colors
ggplot(data = mean_exp_by_time_sex,
       mapping = aes(x = time, y = mean_exp, color = sex)) +
  geom_line() +
  facet_wrap(~ gene, scales = "free_y") +
  theme_bw() +
  theme(panel.grid = element_blank()) +
  scale_color_manual(name = "Gender",  labels = c("F", "M"),
                     values = c("royalblue", "deeppink"))

## Warning in vp$just: partial match of 'just' to 'justification'

6.8 Composing plots

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Faceting is a great tool for splitting one plot into multiple subplots, but sometimes you may want to produce a single figure that contains multiple independent plots, i.e. plots that are based on different variables or even different data frames.

Let’s start by creating the two plots that we want to arrange next to each other:

The first graph counts the number of unique genes per chromosome. We first need to reorder the levels of chromosome_name and filter the unique genes per chromosome. We also change the scale of the y-axis to a log10 scale for better readability.

rna$chromosome_name <- factor(rna$chromosome_name,
                              levels = c(1:19,"X","Y"))

count_gene_chromosome <- rna %>%
    select(chromosome_name, gene) %>%
    distinct() %>%
    ggplot() +
    geom_bar(aes(x = chromosome_name),
             fill = "seagreen",
             position = "dodge",
             stat = "count") +
    labs(y = "log10(n genes)",
         x = "chromosome") +
    scale_y_log10()

count_gene_chromosome

Below, we also remove the legend altogether by setting the legend.position to "none".

exp_boxplot_sex <- ggplot(rna,
                          aes(y = expression_log,
                              x = as.factor(time),
                              color = sex)) +
    geom_boxplot(alpha = 0) +
    labs(y = "Mean gene exp",
         x = "time") +
    theme(legend.position = "none")

exp_boxplot_sex

The patchwork package provides an elegant approach to combining figures using the + to arrange figures (typically side by side). More specifically the | explicitly arranges them side by side and / stacks them on top of each other.

install.packages("patchwork")

library("patchwork")
count_gene_chromosome + exp_boxplot_sex ## or use | instead of +

count_gene_chromosome / exp_boxplot_sex

We can combine further control the layout of the final composition with plot_layout to create more complex layouts:

count_gene_chromosome + exp_boxplot_sex + plot_layout(ncol = 1)

count_gene_chromosome +
    (count_gene_chromosome + exp_boxplot_sex) +
    exp_boxplot_sex +
    plot_layout(ncol = 1)

The last plot can also be created using the | and / composers:

count_gene_chromosome /
    (count_gene_chromosome | exp_boxplot_sex) /
    exp_boxplot_sex

Learn more about patchwork on its webpage or in this video.

Another option is the gridExtra package that allows to combine separate ggplots into a single figure using grid.arrange():

install.packages("gridExtra")

library(gridExtra)
grid.arrange(count_gene_chromosome, exp_boxplot_sex, ncol = 2)

In addition to the ncol and nrow arguments, used to make simple arrangements, there are tools for constructing more complex layouts.

6.9 Exporting plots

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After creating your plot, you can save it to a file in your favorite format. The Export tab in the Plot pane in RStudio will save your plots at low resolution, which will not be accepted by many journals and will not scale well for posters.

Instead, use the ggsave() function, which allows you easily change the dimension and resolution of your plot by adjusting the appropriate arguments (width, height and dpi).

Make sure you have the fig_output/ folder in your working directory.

my_plot <- ggplot(data = mean_exp_by_time_sex,
                  mapping = aes(x = time,
                                y = mean_exp,
                                color = sex)) +
    geom_line() +
    facet_wrap(~ gene, scales = "free_y") +
    labs(title = "Mean gene expression by duration of the infection",
         x = "Duration of the infection (in days)",
         y = "Mean gene expression") +
    guides(color=guide_legend(title="Gender")) +
    theme_bw() +
    theme(axis.text.x = element_text(colour = "royalblue4", size = 12),
          axis.text.y = element_text(colour = "royalblue4", size = 12),
          text = element_text(size = 16),
          panel.grid = element_line(colour="lightsteelblue1"),
          legend.position = "top")
ggsave("fig_output/mean_exp_by_time_sex.png", my_plot, width = 15,
       height = 10)

# This also works for grid.arrange() plots
combo_plot <- grid.arrange(count_gene_chromosome, exp_boxplot_sex,
                           ncol = 2, widths = c(4, 6))
ggsave("fig_output/combo_plot_chromosome_sex.png", combo_plot,
       width = 10, dpi = 300)

Note: The parameters width and height also determine the font size in the saved plot.

6.10 Other packages for visualisation

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ggplot2 is a very powerful package that fits very nicely in our tidy data and tidy tools pipeline. There are other visualisation packages in R that shouldn’t be ignored.

Base graphics

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The default graphics system that comes with R, often called base R graphics is simple and fast. It is based on the painter’s or canvas model, where different output are directly overlaid on top of each other (see figure 6.1). This is a fundamental difference with ggplot2 (and with lattice, described below), that returns dedicated objects, that are rendered on screen or in a file, and that can even be updated.

par(mfrow = c(1, 3))
plot(1:20, main = "First layer, produced with plot(1:20)")
plot(1:20, main = "A horizontal red line, added with abline(h = 10)")
abline(h = 10, col = "red")
plot(1:20, main = "A rectangle, added with rect(5, 5, 15, 15)")
abline(h = 10, col = "red")
rect(5, 5, 15, 15, lwd = 3)

Figure 6.1: Successive layers added on top of each other.

Another main difference is that base graphics’ plotting function try to do the right thing based on their input type, i.e. they will adapt their behaviour based on the class of their input. This is again very different from what we have in ggplot2, that only accepts dataframes as input, and that requires plots to be constructed bit by bit.

par(mfrow = c(2, 2))
boxplot(rnorm(100),
        main = "Boxplot of rnorm(100)")
boxplot(matrix(rnorm(100), ncol = 10),
        main = "Boxplot of matrix(rnorm(100), ncol = 10)")
hist(rnorm(100))
hist(matrix(rnorm(100), ncol = 10))

Figure 6.2: Plotting boxplots (top) and histograms (bottom) vectors (left) or a matrices (right).

The out-of-the-box approach in base graphics can be very efficient for simple, standard figures, that can be produced very quickly with a single line of code and a single function such as plot, or hist, or boxplot, … The defaults are however not always the most appealing and tuning of figures, especially when they become more complex (for example to produce facets), can become lengthy and cumbersome.

6.11 Additional exercises

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► Question

Load the beer consuption data with

library("rWSBIM1207")
data(beers)

Analyse the data to answer the question Do men drink more than women according to age and working status? Now reproduce the figure below.

Figure 6.3: Visualisation of beer consumption, highlighting different patterns of beer consumption in employed and unemployed males and females.

Figure 6.4: Evolution of COVID19 cases.

• Generate a second figure, similar to the one above, for the different regions in France.