Median and quartile on violin plots in ggplot2

Question

I would like to draw some violin plots with ggplot2, but I noticed that median and first and third quartile are not automatically displayed. I believe these plots would be m

Answer 1

geom_violin has an argument draw_quantiles that allows you to specify which quantiles to include. Here is an example of 1st, 2nd, and 3rd quartiles on iris.

require(ggplot2)
ggplot(iris, aes(Species, Sepal.Length)) +
geom_violin(draw_quantiles = c(0.25, 0.5, 0.75))

Answer 2

One way to do this is to just put a thin box plot over the top of it. Here's an example with the iris data:

require(ggplot2)
ggplot(iris,aes(Species,Sepal.Length))+geom_violin()+geom_boxplot(width=.1)

Answer 3

I discovered this from a google search:

First, this Stack Overflow post indicates that you can add stat_summary(fun.y="median", geom="point") to plot the median on a violin plot as a point.

With regard to quartiles, you will likely have to write your own function for the fun.y argument above, as demonstrated on here. E.g.:

median.quartile <- function(x){
    out <- quantile(x, probs = c(0.25,0.5,0.75))
    names(out) <- c("ymin","y","ymax")
    return(out) 
}

The full code might look like this:

require(ggplot2)

median.quartile <- function(x){
  out <- quantile(x, probs = c(0.25,0.5,0.75))
  names(out) <- c("ymin","y","ymax")
  return(out) 
}

ggplot(iris,aes(Species,Sepal.Length))+
  geom_violin()+
  stat_summary(fun.y=median.quartile,geom='point')