ggplot2 boxplot stat_summary text placement by group

Question

In the plot below, I'd like the number of observations (40 in this case) to be overlayed on top of each boxplot. My code below doesn't work when there's a fill aesthetic. The text need to be adjusted horizontally (1 left, 1 center, 1 right in this case) so that they properly overlay their corresponding boxplots.

dt <- data.table(
    x = factor(rep(1:2, each=120))
    , f = rep(letters[1:3], 40)
    , y = c(rnorm(120, 1:3), rnorm(120, 1:3*2))
)
table(dt$x, dt$f)

+--------------+
|      a  b  c |
+--------------+
|   1 40 40 40 |
|   2 40 40 40 |
+--------------+

frequencyAnnotation <- function(x) {
   c(y = (quantile(x, .75, names = F) + median(x))/2, label=length(x))
}
ggplot(dt, aes(x=x, y=y, fill=f)) +
    geom_boxplot() +
    stat_summary(fun.data = frequencyAnnotation, geom='text')

Answer 1

As your boxplots are dodged when you use argument fill=, you have to add position_dodge() to the stat_summary() call.

ggplot(dt, aes(x=x, y=y, fill=f)) +
      geom_boxplot() +
      stat_summary(fun.data = frequencyAnnotation, geom='text', 
                   position = position_dodge(width = 0.75))