2.5 Creating a Box Plot

2.5.1 Problem

You want to create a box plot for comparing distributions.

2.5.2 Solution

To make a box plot (Figure 2.10), use plot() and pass it a factor of x values and a vector of y values. When x is a factor (as opposed to a numeric vector), it will automatically create a box plot:

Box plot with base graphics (left); With multiple grouping variables (right)

Figure 2.10: Box plot with base graphics (left); With multiple grouping variables (right)

If the two vectors are in the same data frame, you can also use the boxplot() function with formula syntax. With this syntax, you can combine two variables on the x-axis, as in Figure 2.10:

With the ggplot2 package, you can get a similar result (Figure 2.11), with geom_boxplot():

Box plot with ggplot() (left); With multiple grouping variables (right)Box plot with ggplot() (left); With multiple grouping variables (right)

Figure 2.11: Box plot with ggplot() (left); With multiple grouping variables (right)

It’s also possible to make box plots for multiple variables, by combining the variables with interaction(), as in Figure 2.11:

Note

You may have noticed that the box plots from base graphics are ever-so-slightly different from those from ggplot2. This is because they use slightly different methods for calculating quantiles. See ?geom_boxplot and ?boxplot.stats for more information on how they differ.

2.5.3 See Also

For more on making basic box plots, see Recipe 6.6.