9.4 Changing the Appearance of Theme Elements

9.4.1 Problem

You want to change the appearance of theme elements.

9.4.2 Solution

To modify a theme, add theme() with a corresponding element_xx object. These include element_line, element_rect, and element_text. The following code shows how to modify many of the commonly used theme properties (Figure 9.9):

library(gcookbook)  # Load gcookbook for the heightweight data set

# Create the base plot
hw_plot <- ggplot(heightweight, aes(x = ageYear, y = heightIn, colour = sex)) +
  geom_point()

# Options for the plotting area
hw_plot +
  theme(
    panel.grid.major = element_line(colour = "red"),
    panel.grid.minor = element_line(colour = "red", linetype = "dashed", size = 0.2),
    panel.background = element_rect(fill = "lightblue"),
    panel.border = element_rect(colour = "blue", fill = NA, size = 2)
  )
#> Warning: The `size` argument of `element_rect()` is deprecated as of ggplot2 3.4.0.
#> ℹ Please use the `linewidth` argument instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.

# Options for the legend
hw_plot +
  theme(
    legend.background = element_rect(fill = "grey85", colour = "red", size = 1),
    legend.title = element_text(colour = "blue", face = "bold", size = 14),
    legend.text = element_text(colour = "red"),
    legend.key = element_rect(colour = "blue", size = 0.25)
  )

# Options for text items
hw_plot +
  ggtitle("Plot title here") +
  theme(
    axis.title.x = element_text(colour = "red", size = 14),
    axis.text.x  = element_text(colour = "blue"),
    axis.title.y = element_text(colour = "red", size = 14, angle = 90),
    axis.text.y  = element_text(colour = "blue"),
    plot.title = element_text(colour = "red", size = 20, face = "bold")
  )

# Options for facets
hw_plot +
  facet_grid(sex ~ .) +
  theme(
    strip.background = element_rect(fill = "pink"),
    strip.text.y = element_text(size = 14, angle = -90, face = "bold")
  )  # strip.text.x is the same, but for horizontal facets

Figure 9.9: Clockwise from top left: modifying theme properties for the plotting area, the legend, the facets, and the text items

9.4.3 Discussion

If you want to use a saved theme and tweak a few parts of it with theme(), the theme() must come after the theme specification. Otherwise, anything set by theme() will be unset by the theme you add:

# theme() has no effect if before adding a complete theme
hw_plot +
  theme(axis.title.x = element_text(colour = "red")) +
  theme_bw()

# theme() works if after a complete theme
hw_plot +
  theme_bw() +
  theme(axis.title.x = element_text(colour = "red", size = 12))

Many of the commonly used theme properties are shown in Table 9.3.

Table 9.3: Theme items that control text appearance in theme()

Name	Description	Element type
text	All text elements	element_text()
rect	All rectangular elements	element_rect()
line	All line elements	element_line()
axis.line	Lines along axes	element_line()
axis.title	Appearance of both axis labels	element_text()
axis.title.x	X-axis label appearance	element_text()
axis.title.y	Y-axis label appearance	element_text()
axis.text	Appearance of tick labels on both axes	element_text()
axis.text.x	X-axis tick label appearance	element_text()
axis.text.y	Y-axis tick label appearance	element_text()
legend.background	Background of legend	element_rect()
legend.text	Legend item appearance	element_text()
legend.title	Legend title appearance	element_text()
legend.position	Position of the legend	"left", "right", "bottom", "top", or two-element numeric vector if you wish to place it inside the plot area (for more on legend placement, see Recipe 10.2)
panel.background	Background of plotting area	element_rect()
panel.border	Border around plotting area	element_rect(linetype="dashed")
panel.grid.major	Major grid lines	element_line()
panel.grid.major.x	Major grid lines, vertical	element_line()
panel.grid.major.y	Major grid lines, horizontal	element_line()
panel.grid.minor	Minor grid lines	element_line()
panel.grid.minor.x	Minor grid lines, vertical	element_line()
panel.grid.minor.y	Minor grid lines, horizontal	element_line()
plot.background	Background of the entire plot	element_rect(fill = "white", colour = NA)
plot.title	Title text appearance	element_text()
strip.background	Background of facet labels	element_rect()
strip.text	Text appearance for vertical and horizontal facet labels	element_text()
strip.text.x	Text appearance for horizontal facet labels	element_text()
strip.text.y	Text appearance for vertical facet labels	element_text()