How to add standard error bars to a box and whisker plot using ggplot2?

Question

I'm trying to add standard error bars to my data similar to the ones seen on the box plots near the end of the answer on this question: https://stats.stackexchange.com/questions/8137/how-to-add-horizontal-lines-to-ggplot2-boxplot

I am using the PlantGrowth dataset, which looks like this (except 30 rows long in total):

    weight    group
1   4.17      ctrl
2   5.58      ctrl
3   4.81      trt1
4   4.17      trt1
5   6.31      trt2
6   5.12      trt2

I have produced this plot

with the following code

 ggplot(PlantGrowth, aes(group, weight))+
stat_boxplot(geom='errorbar', linetype=1, width=0.5)+  #whiskers
geom_boxplot(outlier.shape=1)+    
stat_summary(fun.y=mean, geom="point", size=2)   #dot for the mean

I don't know how to add an error bar for each plot based on the variation within that factor variable. I have added +geom_errorbar(aes(x=group, ymin=mean-sd, ymax=mean+sd)) but it returns the error "Error in mean - sd : non-numeric argument to binary operator"

Any help would be appreciated. Thank you

Answer 1

There is a mean_se function in ggplot2 which does exactly what you want.

library(ggplot2)
ggplot(PlantGrowth, aes(group, weight))+
  stat_boxplot( aes(group, weight), 
    geom='errorbar', linetype=1, width=0.5)+  #whiskers
  geom_boxplot( aes(group, weight),outlier.shape=1) +    
  stat_summary(fun.y=mean, geom="point", size=2) + 
  stat_summary(fun.data = mean_se, geom = "errorbar")