Using geom_text() with italics and line breaks in ggplot2

When creating a geospatial map using the ggplot2 package, one common requirement is to display additional information on top of each tile. In this case, we want to show both the beta coefficient and the p-value for each tile. However, we also need to format these values in a specific way: italicized letter followed by the p-value on a new line.

In this article, we’ll explore how to achieve this using geom_text() from ggplot2. We’ll go over various approaches, including using HTML tags and markdown syntax, as well as using other packages like ggtext for more flexibility.

Introduction to geom_text()

geom_text() is a built-in layer in ggplot2 that allows us to add labels or text annotations to our geospatial map. It takes several arguments, including the mapping aesthetic ( aes() ) which specifies what variables are mapped to each element of the plot. The most important argument for this use case is label, which specifies the text that will be displayed.

By default, geom_text() assumes that we want to display the label as plain text without any special formatting. However, in our case, we need to add an italicized letter and a line break (new line) to the p-value string.

Attempting to use parse = F with expression()

One approach is to try using the parse = F argument, which allows us to pass raw R expressions directly into the label. We can wrap the text we want to display in an italic() function from the utils package and use a line break (\n) to create a new line.

ggplot(df, aes(x = x, y = y)) +
  geom_tile() +
  geom_text(
    mapping = aes(label = paste(value, "\n", expression(italic("p")), "==", p)),
    parse = F,
    color = "white"
  )

However, when we run this code, we encounter an error message indicating that there’s a problem with the parsing of the label. The parse = F argument allows us to avoid this issue by passing raw R expressions, but in this case, it seems like the expression() function is not being called correctly.

Using HTML Tags

Another approach is to use HTML tags to create the desired formatting. Specifically, we can wrap the text we want to display in <i> (italic) tags and use the <br> tag to create a line break.

ggplot(df, aes(x = x, y = y)) +
  geom_tile() +
  geom_text(
    mapping = aes(label = paste(value, "\n", "*p* =", p)),
    parse = T,
    color = "white"
  )

Unfortunately, this approach doesn’t quite work as expected. The geom_text() layer in ggplot2 has a default behavior of adding a space between the label and its mapping aesthetic. This causes an unexpected symbol error when trying to wrap the text in <i> tags.

Using ggtext::geom_richtext()

Fortunately, there’s another package called ggtext that provides additional functionality for styling text in ggplot2 plots. Specifically, it offers a geom_richtext() layer that allows us to add styled text using HTML or markdown syntax.

To use this approach, we first need to install the ggtext package and load it in our R session.

library(ggplot2)
library(ggtext)

Then, we can wrap the text we want to display in markdown syntax (specifically, <i>) tags and add a line break using the \n escape sequence. We also need to set the fill color and label.color arguments to NA to ensure that the label box is not displayed around our styled text.

df <- data.frame(x = letters[1:10], y = letters[11:20], value = c(1:10), p = c((1:10) / 10))

library(ggplot2)
library(ggtext)

ggplot(df, aes(x = x, y = y)) +
  geom_tile() +
  ggtext::geom_richtext(
    aes(label = paste(value, "\n", "*p*", "=", p)),
    color = "white",
    fill = NA,
    label.color = NA
  )

This approach works as expected and produces the desired output: italicized text followed by a line break, with the p-value displayed in a new line.

Conclusion

In conclusion, using geom_text() to add styled text in ggplot2 plots can be challenging due to its limited formatting options. However, by exploring alternative approaches such as HTML tags and markdown syntax, we can achieve more flexibility in styling our labels. The use of the ggtext package provides an additional layer of functionality that makes it easier to create complex layouts and styles in ggplot2 plots.

We hope this article has provided a useful example of how to add styled text to your ggplot2 maps using various approaches. If you have any questions or need further assistance, feel free to ask!

Last modified on 2025-02-24