Customizing X-Axis Labels in ggplot2 Facets Using Grob Structure

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In this article, we’ll explore how to customize the x-axis labels for different facets in a ggplot2 plot that uses facet_wrap(). We’ll delve into the details of how ggplot2 renders plots and show you how to manipulate the plot’s grob structure to achieve your desired layout.

Background

When creating a ggplot2 plot with multiple facets, the plot is rendered as a sequence of graphical objects (grobs). Each grob represents a component of the plot, such as a panel, axis, or strip. The facet_wrap() function creates new facets by duplicating panels from the original plot and then reassembling them into separate subplots.

In our example, we have a plot with four facets, each representing a different variable in our dataset. We want to customize the x-axis labels for the first facet to display only the year (2018, 19, 20, 21), while keeping the same labels for the other three facets.

Using dplyr and ggplot2

To achieve this customization, we can use the dplyr library to manipulate our data and then create a new plot with the desired x-axis labels.

library(ggplot2)
library(dplyr)

df <- structure(list(date = structure(c(17532, 17897, 18262, 18628, 
                      17532, 17897, 18262, 18628, 17532, 17897, 18262, 18628), class = "Date"), 
                     var = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 3L, 3L, 
                     3L, 3L), .Label = c("a", "b", "c"), class = "factor"), value = c(-1.09243979230319, 
                    1.61989681830401, 0.0814424200676222, -0.408078030248257, 
                    -0.904237506821678, 0.787027860989217, 0.706295579996191, 
                    -0.223410122960503, 0.0764830701612793, 0.927863681429729, 
                    -0.56458790218662, -0.954306740205979)), class = "data.frame", row.names = c(NA, -12L))

ggplot(df, aes(x = date, y = value)) +
  geom_bar(stat = 'identity') +
  scale_x_date(breaks = unique(df$date), labels = c(2018, 19:21)) +
  facet_wrap(~var)

Accessing the grob structure

To customize the x-axis labels for each facet, we need to access the plot’s grob structure. We can use the ggplotGrob() function from the grid package to get the plot as a sequence of graphical objects.

library(grid)

gt <- ggplotGrob()
print(gt)

This will print the entire plot, including all grobs.

Identifying the axis-b grobs

From the output, we can see that the x-axis labels belong to grob #8 to #10, which are axis-b-1, axis-b-2, and axis-b-3.

gt$grobs[[8]]$children[[2]]$grobs[[2]]$children[[1]]$label

Modifying the axis-b grobs

We can modify these grobs to change the x-axis labels for each facet. To do this, we need to access the label attribute of each grob and update its value.

gt$grobs[[8]]$children[[2]]$grobs[[2]]$children[[1]]$label <- c(2018, 19:21)
gt$grobs[[9]]$children[[2]]$grobs[[2]]$children[[1]]$label <- ""

This will change the x-axis labels for the first facet to display only the year (2018, 19, 20, 21), while keeping the same labels for the other three facets.

Rendering the updated plot

Finally, we can render the updated plot using the grid.newpage() and grid.draw() functions from the grid package.

grid.newpage()
grid.draw(gt)

This will display the updated plot with customized x-axis labels for each facet.

Last modified on 2025-04-03

Controlling x-labels in facet_wrap ggplot2