How to modify whiskers of a boxplot in ggplot2?

Problem

We want to modify the colour of the whiskers in a boxplot using ggplot2. In our example, the default colour (black) to red.

library(ggplot2)
p <- ggplot(mtcars, aes(factor(cyl), mpg, fill = factor(am)))
p + geom_boxplot()

Solution

• Option 1

We plot error bars in red and then superimpose the boxplots in black.

library(ggplot)
p + stat_boxplot(
  geom = "errorbar",
  colour = "red",
  width = 0,
  position = position_dodge(0.75)
) +
  geom_boxplot(coef = 0, outlier.shape = NA)


• Option 2

We superimpose two boxplots on top of each other. The first one with red borders and the secong one without whiskers in black.

p + geom_boxplot(color="red") + 
  geom_boxplot(aes(ymin=..lower.., ymax=..upper..)) 

References

• Box and whiskers plot.
• stackoverflow