Centering Plot Titles in ggplot2

ggplot2 is a powerful and versatile R package for creating data visualizations. A common task when building plots is controlling the alignment of elements, including the plot title. By default, ggplot2 left-aligns the plot title. However, centering the title often improves the visual appeal and balance of a plot. This tutorial explains how to center plot titles effectively using different approaches.

Understanding the Default Behavior

Prior to version 2.2.0, ggplot2 centered plot titles by default. However, with version 2.2.0 and later, the default behavior changed to left-alignment. This change was implemented to better accommodate subtitles which frequently appear below the main title. If you’re upgrading from an older version of ggplot2, you might notice this change in title alignment.

Using `theme()` to Center the Title

The most common and direct method to center the plot title is to use the theme() function. The theme() function allows you to customize various non-data elements of your plot. To center the title, you modify the plot.title element within theme().

Specifically, the hjust (horizontal justification) argument controls the alignment. Setting hjust = 0.5 centers the text. Here’s how you do it:

library(ggplot2)

# Sample data
dat <- data.frame(
  time = factor(c("Lunch", "Dinner"), levels = c("Lunch", "Dinner")),
  total_bill = c(14.89, 17.23)
)

# Create the plot with a centered title
ggplot(data = dat, aes(x = time, y = total_bill, fill = time)) +
  geom_bar(colour = "black", fill = "#DD8888", width = .8, stat = "identity") +
  guides(fill = FALSE) +
  xlab("Time of day") +
  ylab("Total bill") +
  ggtitle("Average bill for 2 people") +
  theme(plot.title = element_text(hjust = 0.5))

In this example, theme(plot.title = element_text(hjust = 0.5)) adds the centering customization to the plot.

Setting a Default Theme for All Plots

If you find yourself repeatedly centering titles in multiple plots, you can set a default theme to avoid specifying theme(plot.title = element_text(hjust = 0.5)) every time. Use theme_update():

theme_update(plot.title = element_text(hjust = 0.5))

# Now, any plot created will have a centered title by default.
ggplot(data = dat, aes(x = time, y = total_bill, fill = time)) +
  geom_bar(colour = "black", fill = "#DD8888", width = .8, stat = "identity") +
  guides(fill = FALSE) +
  xlab("Time of day") +
  ylab("Total bill") +
  ggtitle("Another plot with a centered title")

This approach modifies the global ggplot2 theme settings, so all subsequent plots will automatically have centered titles. To revert to the default ggplot2 theme, you can either restart your R session or use theme_set(theme_gray()).

Creating a Custom Theme

For more complex customization, or if you want to apply a consistent set of visual elements across multiple plots, you can define a custom theme. This allows you to encapsulate all your customizations into a single reusable object.

personal_theme <- theme(plot.title = element_text(hjust = 0.5))

# Apply the custom theme to a plot
ggplot(data = dat, aes(x = time, y = total_bill, fill = time)) +
  geom_bar(colour = "black", fill = "#DD8888", width = .8, stat = "identity") +
  guides(fill = FALSE) +
  xlab("Time of day") +
  ylab("Total bill") +
  ggtitle("Plot with a custom theme") +
  personal_theme

Using the `ggeasy` Package

The ggeasy package provides a convenient function, easy_center_title(), to easily center plot titles. This is a more concise alternative to using theme() directly.

library(ggeasy)

ggplot(data = dat, aes(x = time, y = total_bill, fill = time)) +
  geom_bar(colour = "black", fill = "#DD8888", width = .8, stat = "identity") +
  guides(fill = FALSE) +
  xlab("Time of day") +
  ylab("Total bill") +
  ggtitle("Average bill for 2 people") +
  ggeasy::easy_center_title()

Note that you might need to install the development version of ggeasy from GitHub to use easy_center_title() if you’re using an older version. You can install it using remotes::install_github("jonocarroll/ggeasy").

Conclusion

Centering plot titles in ggplot2 is a straightforward process. Whether you choose to use the theme() function, create a custom theme, or leverage the convenience of the ggeasy package, you have several options to achieve the desired visual effect. Choose the method that best suits your needs and coding style for creating professional and visually appealing plots.