r - `ggplot2`: label values of barplot that uses `fun.y="mean"` of `stat_summary`

Question

If I use ggplot2's stat_summary() to make a barplot of the average number of miles per gallon for 3-, 4-, and 5-geared cars, for example, how can I label each of the bars with the average value for mpg?

library(ggplot2)
CarPlot <- ggplot() +
  stat_summary(data= mtcars,
               aes(x = factor(gear),
                   y = mpg,
                   fill = factor(gear)
                   ),
               fun.y="mean",
               geom="bar"
               )
CarPlot

I know that you can normally use geom_text(), but I'm having trouble figuring out what to do in order to get the average value from stat_summary().

Answer 1

You should use the internal variable ..y.. to get the computed mean.

library(ggplot2)
CarPlot <- ggplot(data= mtcars) +
               aes(x = factor(gear),
                   y = mpg)+
      stat_summary(aes(fill = factor(gear)), fun.y=mean, geom="bar")+
      stat_summary(aes(label=round(..y..,2)), fun.y=mean, geom="text", size=6,
             vjust = -0.5)
CarPlot

but probably it is better to aggregate beforehand.