annotate boxplot in ggplot2

Question

I\'ve created a side-by-side boxplot using ggplot2.

p <- ggplot(mtcars, aes(x=factor(cyl), y=mpg))
p + geom_boxplot(aes(fill=factor(cyl)))

Answer 1

The most succinct way I can think of is to use stat_summary. I've also mapped the labels to a color aesthetic, but you can, of course, set the labels to a single color if you wish:

ggplot(mtcars, aes(x=factor(cyl), y=mpg, fill=factor(cyl))) + 
  geom_boxplot(width=0.6) +
  stat_summary(geom="text", fun.y=quantile,
               aes(label=sprintf("%1.1f", ..y..), color=factor(cyl)),
               position=position_nudge(x=0.33), size=3.5) +
  theme_bw()

In the code above we use quantile as the summary function to get the label values. ..y.. refers back to the output of the quantile function (in general, ..*.. is a ggplot construction for using values calculated within ggplot).