Understanding geom_text() Position Change with Different Axis Span or Length: A Solution

Introduction

The geom_text() function in ggplot2 is a powerful tool for adding labels to data points. However, it can sometimes behave unexpectedly when the axis span or length changes. In this article, we will explore the issue and provide solutions using the ggtext package.

Problem Description

Consider the following code:

library("ggplot2")
dev.new()

ggplot(mtcars, aes(x=mpg, y=hp)) +
    geom_point() +
    geom_text(label=rownames(mtcars), nudge_x=5, nudge_y=5) 
    
mtcars_mod <- rbind.data.frame(mtcars, c(80, 6, 123.0, 111, 5.14, 1.7, 22.1, 2, 1, 3, 1))
rownames(mtcars_mod) <- c(rownames(mtcars), "Fake Car")


dev.new()

ggplot(mtcars_mod, aes(x=mpg, y=hp)) +
    geom_point() +
    geom_text(label=rownames(mtcars_mod), nudge_x=5, nudge_y=5)

When we run this code, we get two different plots. The second plot has a fake car added, which changes the x-axis span compared to the first plot. If we use the same nudge value for both plots, the labels are placed in different positions with respect to the data points. This is because the scale of the axis affects the position of the labels.

Solution: Using geom_richtext()

One option to address this issue is to use the geom_richtext() function from the ggtext package. This function allows us to “shift” the labels by setting a margin in units such as mm or pt, which is independent of the scale.

Here’s an updated code snippet that demonstrates how to use geom_richtext():

library(ggplot2)
library(ggtext)

mtcars_mod <- rbind.data.frame(mtcars, c(80, 6, 123.0, 111, 5.14, 1.7, 22.1, 2, 1, 3, 1))
rownames(mtcars_mod) <- c(rownames(mtcars), "Fake Car")

ggplot(mtcars, aes(x = mpg, y = hp)) +
  geom_point() +
  geom_richtext(
    label = rownames(mtcars),
    label.margin = unit(c(0, 0, 5, 5), "mm"),
    hjust = 0,
    fill = NA, label.colour = NA
  )

ggplot(mtcars_mod, aes(x = mpg, y = hp)) +
  geom_point() +
  geom_richtext(
    label = rownames(mtcars_mod),
    label.margin = unit(c(0, 0, 5, 5), "mm"),
    hjust = 0,
    fill = NA, label.colour = NA
  )

In this updated code snippet, we use the geom_richtext() function to add labels to the data points. We set the label.margin parameter to specify a margin of 5 mm in both directions (top and bottom) from the edge of the plot area. The hjust = 0 parameter centers the label horizontally.

By using geom_richtext(), we can ensure that the labels are placed consistently with respect to the data points, regardless of changes in the axis span or length.

Additional Tips and Variations

Here are some additional tips and variations you may want to consider:

• Customizing the Label Appearance: You can customize the label appearance by adding additional parameters to the geom_richtext() function. For example, you can change the font size, color, or style using the fontface and fontsize parameters.

ggplot(mtcars, aes(x = mpg, y = hp)) + geom_point() + geom_richtext( label = rownames(mtcars), label.margin = unit(c(0, 0, 5, 5), “mm”), hjust = 0, fontface = “bold”, fontsize = 12, fill = NA, label.colour = NA )

*   **Using Other Units**: You can use other units such as `pt` (point) or `in` (inch) instead of `mm` to specify the margin.
    ```markdown
ggplot(mtcars, aes(x = mpg, y = hp)) +
  geom_point() +
  geom_richtext(
    label = rownames(mtcars),
    label.margin = unit(c(0, 0, 5, 5), "pt"),
    hjust = 0,
    fill = NA, label.colour = NA
  )

• Combining with Other Geoms: You can combine the geom_richtext() function with other geoms to create more complex plots. For example, you can add a line geom or a scatter plot using geom_line() and geom_point().

ggplot(mtcars, aes(x = mpg, y = hp)) + geom_point() + geom_richtext( label = rownames(mtcars), label.margin = unit(c(0, 0, 5, 5), “mm”), hjust = 0, fill = NA, label.colour = NA ) + geom_line(aes(x = mpg, y = hp))

By following these tips and variations, you can create more complex and informative plots using the `geom_richtext()` function.

Last modified on 2025-02-17