One of the most useful techniques in data visualization is rendering groups of data alongside each other, making it easy to compare the groups. With ggplot2, one way to do this is by mapping a discrete variable to an aesthetic, like x
shape. Another way of doing this is to create a subplot for each group and draw the subplots side by side.
These kinds of plots are known as Trellis displays. They’re implemented in the lattice package as well as in the ggplot2 package. In ggplot2, they’re called facets. In this chapter I’ll explain how to use them.