Notched boxplot - wch/ggplot2 GitHub Wiki

I've added an option for notches to geom_boxplot. There are two new parameters for geom_boxplot:

  • notch: TRUE / FALSE
  • notchwidth: the relative width of the notch to the overall width (default .5)

Some examples:

set.seed(110)
dat <- data.frame(x=LETTERS[1:3], y=round(rnorm(90),2))

# Notched box plot
ggplot(dat, aes(x=x, y=y)) + geom_boxplot(notch=TRUE)


# Can set notch width
ggplot(dat, aes(x=x, y=y)) + geom_boxplot(notch=TRUE, notchwidth=.2)


# Manually-specified positions
ggplot(NULL, aes(x = 1, y=NULL, ymin = 0, lower = 25, middle = 50,
                 upper = 75, ymax = 100, notchupper=60, notchlower=40)) +
  geom_boxplot(stat="identity", notch=TRUE) 


# Gives a warning if notches go past hinges
ggplot(mtcars, aes(factor(cyl), mpg)) + geom_boxplot(notch=TRUE) + ylim(10,35)
# Warning messages:1: In get(x, envir = this, inherits = inh)(this, ...) :
#   notch went outside hinges. Try setting notch=FALSE.
# 2: In get(x, envir = this, inherits = inh)(this, ...) :
#   notch went outside hinges. Try setting notch=FALSE.


# Regular boxplot function gives similar warning
boxplot(mpg ~ cyl, mtcars, notch=TRUE)
# Warning message:
# In bxp(list(stats = c(21.4, 22.8, 26, 30.4, 33.9, 17.8, 18.65, 19.7,  :
#   some notches went outside hinges ('box'): maybe set notch=FALSE

(You may have noticed that the outliers are different between geom_boxplot and the base boxplot. This is because geom_boxplot uses the 25% and 75% quantiles to calculate hinge positions, while boxplot does something slightly different, depending on the exact number of observations. See ?boxplot.stats for more information.)

The notch option also supports weighted boxplots. Normally the notch size is:

median +- 1.58*IQR*sqrt(n)

Where n is the number of observations. Weighted boxplots require a change to the calculation of n, so it uses the sum of all weights at observations where weight and y values are not NA. For example, three unweighted observations at a given y value are the same as one observation with weight=3, in terms of calculating notch size.

Adding the notch required some changes to geom_crossbar; instead of drawing the body with a GeomRect, it now uses GeomPolygon.