Boxplots in Base R

This section introduces the functionality of the base-R function boxplot. Note that for some data formats, the plot function with x being a factor variable will also work.

R base box plots: boxplot()

Draw a box plot of teeth length (len):

Basic box plots

# Box plot of one variable
boxplot(ToothGrowth$len)
# Box plots by groups (dose)
# remove frame
boxplot(len ~ dose, data = ToothGrowth, frame = FALSE)
# Horizontal box plots
boxplot(len ~ dose, data = ToothGrowth, frame = FALSE,
        horizontal = TRUE)
# Notched box plots
boxplot(len ~ dose, data = ToothGrowth, frame = FALSE,
        notch = TRUE)

Notch is used to compare groups. In the notched boxplot, if two boxes' notches do not overlap this is "strong evidence" their medians differ (Chambers et al., 1983, p. 62).