R ggplot2: Add means as horizontal line in a boxplot

boxplot, ggplot2, r

Solution

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")

Problem

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!

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