R ggplot2: Add means as horizontal line in a boxplot

Question

I have created a boxplot using ggplot2:

library(ggplot2)

dat <- data.frame(study = c(rep('a',50),rep('b',50)), 
                  FPKM = c(rnorm(1:50),rnorm(1:50)))

ggplot(dat, aes(x = study, y = FPKM)) + geom_boxplot()

The boxplot shows the median as a horizontal line across each box.

How do I add a dashed line to the box representing the mean of that group?

Thanks!

Answer 1

You can add horizontal lines to plots by using stat_summary with geom_errorbar. The line is horizontal because the y minimum and maximum are set to be the same as y.

ggplot(dat, aes(x = study, y = FPKM)) + 
    geom_boxplot() +
    stat_summary(fun.y = mean, geom = "errorbar", aes(ymax = ..y.., ymin = ..y..),
                 width = .75, linetype = "dashed")