Understanding the Issue with Reordered Legends in ggplot2

As a data analyst or scientist, you’ve likely worked with various visualization libraries to communicate complex insights. One such library is ggplot2, a powerful tool for creating high-quality statistical graphics. However, when it comes to customizing legends, especially after reordering them, things can get tricky.

In this article, we’ll delve into the world of ggplot2 legends and explore why reordering the legend causes issues with drawing new legends. We’ll also provide a step-by-step solution to resolve this issue.

A Brief Introduction to ggplot2 Legends

When creating a ggplot2 plot, you can customize various aspects of the visualization, including the color palette. One such aspect is the legend, which displays colors and labels corresponding to different data groups.

The scale_fill_discrete function allows you to specify discrete breaks for fill mapping. By default, ggplot2 creates two legends: one above the plot (default) and another below the plot. However, when reordering the legend breaks, it can lead to unexpected behavior.

The Problem with Reordered Legends

Let’s examine the code snippet provided in the Stack Overflow post:

p1 <- ggplot(data = dframe1, mapping = aes(x = x, y = y)) +
  geom_density_line(stat = "identity", size=.5, alpha=0.3, aes(color=Electrode, fill=Electrode)) +
  scale_fill_discrete(breaks=c("Fz","Cz","Pz")) +
  guides(colour = FALSE) +
  geom_vline(xintercept = 0) +
  xlab("values") +
  xlim(-2, 10) +
  ylab("density") +
  ylim(0, .7) +
  labs(title="Interval")

Notice the guides(colour = FALSE) line. This adds a guide that prevents color changes in the legend.

However, even with this adjustment, reordering the legend breaks results in multiple legends being drawn.

Understanding aes() and geom_density_line()

The key to solving this issue lies in understanding how aes() and geom_density_line() interact with each other.

When using geom_density_line(), ggplot2 automatically generates a line plot for density estimation. The aes() function within this context is used to map variables to the aesthetic attributes (color, fill, etc.).

By default, geom_density_line() maps the color variable directly to the fill attribute, which creates an additional legend. This happens because the fill mapping is not explicitly set.

The Solution: Adding guides()

To resolve the issue of multiple legends being drawn after reordering the legend breaks, we need to modify the scale_fill_discrete function to suppress the creation of a second legend.

The solution lies in adding the following line:

+ guides(fill = FALSE)

This tells ggplot2 not to create an additional legend for fill mapping.

Complete Example Code

Here’s the complete code with all necessary adjustments:

p1 <- ggplot(data = dframe1, mapping = aes(x = x, y = y)) +
  geom_density_line(stat = "identity", size=.5, alpha=0.3, aes(color=Electrode, fill=Electrode)) +
  scale_fill_discrete(breaks=c("Fz","Cz","Pz")) +
  guides(fill = FALSE) +
  guides(colour = FALSE) +
  geom_vline(xintercept = 0) +
  xlab("values") +
  xlim(-2, 10) +
  ylab("density") +
  ylim(0, .7) +
  labs(title="Interval")

Conclusion

Reordering legend breaks in ggplot2 can lead to unexpected behavior when trying to suppress the creation of multiple legends. By understanding how aes() and geom_density_line() interact with each other, we can identify the root cause of this issue.

Using the guides() function is key to resolving this problem by suppressing fill mapping. By adding fill = FALSE to both color and fill guides, we ensure that only a single legend appears after reordering the breaks.

I hope this in-depth explanation has helped clarify why reordering legends causes issues in ggplot2 and provided a clear solution for resolving this issue!

Last modified on 2024-06-18