You want to add markers for the mean to a box plot.

Use stat_summary(). The mean is often shown with a diamond, so we’ll use shape 23 with a white fill. We’ll also make the diamond slightly larger by setting size=3 (Figure 6-20):

library(MASS) # For the data set

ggplot(birthwt, aes(x=factor(race), y=bwt)) + geom_boxplot() +
    stat_summary(fun.y="mean", geom="point", shape=23, size=3, fill="white")

Figure 6-20. Mean markers on a box plot

The horizontal line in the middle of a box plot displays the median, not the mean. For data that is normally distributed, the median and mean will be about the same, but for skewed data these values will differ.