Matt Bannert Matt Bannert - 1 month ago 11
R Question

Can I get boxplot notches in ggplot2?

Yes, I know it's been around, I've also found Hadley's answer on google groups that there is no notches yet for

ggplot2
boxplots. So my question is twofold: Has this changed (there's a native implementation of notches already) and if not is there something one could do about it.

I mean I do not need the notch optic, representing the confidence bounds by some shaded area that is suitably placed in another layer over the boxplot, would look nice, too.

Also added a screenshot because I heard a graphics question is never complete without the graphic
enter image description here

Answer

Update In addition to the options detailed below, version 0.9.0 of ggplot2 includes this feature in geom_boxplot. Examining ?geom_boxplot reveals a notch and notchwidth argument:

+ geom_boxplot(notch = TRUE, notchwidth = 0.5)

Not elegant graphics but here is an example:

# confidence interval calculated by `boxplot.stats`
f <- function(x) {
    ans <- boxplot.stats(x)
    data.frame(ymin = ans$conf[1], ymax = ans$conf[2])
}

# overlay plot (upper panel below)
p <- ggplot(iris, aes(Species, Sepal.Length)) + geom_boxplot() +
  stat_summary(fun.data = f, geom = "linerange", colour = "skyblue", size = 5)
p

# base graphics (lower panel below)
boxplot(Sepal.Length ~ Species, data = iris, notch = TRUE)

you can change the apparence of CI bar by tweaking the arguments of stat_summary.

enter image description here enter image description here

crossbar version:

f <- function(x) {
  ans <- boxplot.stats(x)
  data.frame(ymin = ans$conf[1], ymax = ans$conf[2], y = ans$stats[3])
}

p <- ggplot(iris, aes(Species, Sepal.Length)) + 
  geom_boxplot(width = 0.8) +
  stat_summary(fun.data = f, geom = "crossbar", 
    colour = NA, fill = "skyblue", width = 0.8, alpha = 0.5)
p

enter image description here