The Problem: Tiny Legend Keys

When plotting many data points, we often decrease the size to avoid overplotting. However, this makes the legend keys tiny:

library(ggplot2)

p <- ggplot(iris, aes(Sepal.Length, Sepal.Width, color = Species)) +
  geom_point(size = 0.5) + 
  theme_minimal() +
  labs(title = "Tiny Legend Keys")

p

The Solution: Using override.aes

By using guides(), we can “override” the aesthetics of the legend. This allows us to keep the small points in the plot while making the legend keys large and clear.

If your legend is for the color aesthetic:

p_fixed <- p + 
  guides(color = guide_legend(override.aes = list(size = 5))) +
  labs(title = "Fixed Legend Keys (Size 5)")

p_fixed

If you were using fill (e.g., in a bar plot or boxplot), you would simply change color to fill:

guides(fill = guide_legend(override.aes = list(size = 5)))

Adjusting Legend Text and Title Size

Note that override.aes only affects the symbols inside the legend. To change the size of the labels or the legend title, you should use the theme() function:

p_fixed + 
  theme(
    legend.text = element_text(size = 12),   # Legend labels
    legend.title = element_text(size = 14, face = "bold") # Legend title
  )

Summary

• Use guides(color = guide_legend(override.aes = list(size = ...))) to change the symbol size.
• Use theme(legend.text = element_text(size = ...)) to change the text size.