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 annota

Answer 1

Alternatively, the package cowplot has the handy annotation function draw_label(). When used in combination with ggdraw(), can annotate anywhere on the canvas/sheet with the coordinates ranging from 0 to 1 (relative to the entire canvas). The function cowplot::draw_label() uses ggplot2::annotation_custom() under the hood.

library(ggplot2)
library(cowplot)
#> Warning: package 'cowplot' was built under R version 3.5.2
#> 
#> Attaching package: 'cowplot'
#> The following object is masked from 'package:ggplot2':
#> 
#>     ggsave

# Revert to default theme; see https://stackoverflow.com/a/41096936/5193830
theme_set(theme_grey())

p <- ggplot(mtcars, aes(x = hp, y = mpg)) +
  geom_point() +
  facet_grid(. ~ cyl)

ggdraw(p) + draw_label("XX", x = 0.02, y = 0.97)

^{Created on 2019-01-14 by the reprex package (v0.2.1)}