Annotate outside plot area once in ggplot with facets

Question

I want to add an annotation outside the plotting area in a faceted ggplot. I can get the annotation that I want, but it's repeated for each facet. How can I get this annotation to appear only once?

E.g., to annotate "XX" once in the top left hand corner I can use:

library("ggplot2")
ggplot(mtcars, aes(x = hp, y = mpg)) +
  geom_point() +
  facet_grid(.~cyl ) + 
  annotate("text", x = -20, y = 36, label = "XX") +
  coord_cartesian(xlim = c(50, 350), ylim = c(10, 35), clip = "off")

However this actually annotates it to the top left of each facet.

How can I get this to only appear once?

Answer 1

It's in fact very easy, just have a vector of labels, where the ones you don't want to plot are the empty string "".

library("ggplot2")

ggplot(mtcars, aes(x = hp, y = mpg)) +
  geom_point() +
  annotate("text", x = -20, y = 36, label = c("XX", "", "")) +
  facet_grid(.~cyl ) + 
  coord_cartesian(xlim = c(50, 350), ylim = c(10, 35), clip = "off")

Answer 2

You can put a single tag label on a graph using tag in labs().

ggplot(mtcars, aes(x = hp, y = mpg)) +
     geom_point() +
     facet_grid(.~cyl ) + 
     labs(tag = "XX") +
     coord_cartesian(xlim = c(50, 350), ylim = c(10, 35), clip = "off")

This defaults to "top left", though, which may not be what you want. You can move it around with the theme element plot.tag.position, either as coordinates (between 0 and 1 to be in plot space) or as a string like "topright".

ggplot(mtcars, aes(x = hp, y = mpg)) +
     geom_point() +
     facet_grid(.~cyl ) + 
     labs(tag = "XX") +
     coord_cartesian(xlim = c(50, 350), ylim = c(10, 35), clip = "off") +
     theme(plot.tag.position = c(.01, .95))