9.14 Facets

• Facets are used to splits a plot into subplots that each display one subset of the data based on a categorical variable.

• To facet your plot with the combination of two variables, switch from facet_wrap(), which we learned about in chapter 2, to facet_grid(), which uses a double sided formula, rows~cols.

par(mar = c(4, 4, .1, .1))

# Left
ggplot(mpg, aes(x = displ, y = hwy)) + 
  geom_point() + 
  facet_wrap(~cyl)


# Right
ggplot(mpg, aes(x = displ, y = hwy)) + 
  geom_point() + 
  facet_grid(drv ~ cyl)