16.4 Missing faceting variables

If you are using faceting on a plot with multiple datasets, what happens when one of those datasets is missing the faceting variables? This situation commonly arises when you are adding contextual information that should be the same in all panels. For example, imagine you have a spatial display of disease faceted by gender. What happens when you add a map layer that does not contain the gender variable? Here ggplot will do what you expect: it will display the map in every facet: missing faceting variables are treated like they have all values.

Here’s a simple example. Note how the single red point from df2 appears in both panels.

df1 <- data.frame(x = 1:3, y = 1:3, gender = c("f", "f", "m"))
df2 <- data.frame(x = 2, y = 2)

ggplot(df1, aes(x, y)) + 
  geom_point(data = df2, colour = "red", size = 2) + 
  geom_point() + 
  facet_wrap(~gender)

This technique is particularly useful when you add annotations to make it easier to compare between facets, as shown in the next section.