Adding multiple annotations when faceting - r

Question

I'm having trouble adding multiple annotations (using vectors) to a plot with facets.
For example:
library(tidyverse) # ggplot2_3.3.0
tibble(t = 1:100) %>%
crossing(id = LETTERS[1:2]) %>%
group_by(id) %>%
mutate(y = cumsum(rnorm(n()))) %>%
ggplot(aes(t, y)) + # perhaps add `group = id` if you don't facet by `id`
facet_wrap(vars(id)) + # (1)
annotate('rect', xmin = 20, xmax = 30, ymin = -Inf, ymax = Inf, fill = 'grey60') + # (2)
annotate('rect', xmin = 30, xmax = 40, ymin = -Inf, ymax = Inf, fill = 'grey70') + # (2)
annotate('rect', xmin = 40, xmax = 50, ymin = -Inf, ymax = Inf, fill = 'grey80') + # (2)
annotate('rect', xmin = 50, xmax = 60, ymin = -Inf, ymax = Inf, fill = 'grey90') + # (2)
# annotate('rect', ymin = -Inf, ymax = Inf, # (3)
# xmin = seq(20, by=10, len=4), # (3)
# xmax = seq(30, by=10, len=4), # (3)
# fill = paste0('grey', seq(60, by=10, len=4))) + # (3)
geom_line() +
theme_light()
The above code produces the desired plot (in particular, I want the same annotation on all facets). However, the annotate command is repeated four times; furthermore the help page for annotate says "the properties of the geoms are ... passed in as vectors".
So a natural thing to try is to comment out lines (2), and uncomment lines (3).
Unfortunately this generates the error
Error: Aesthetics must be either length 1 or the same as the data (8): fill
Note that if, in addition, you comment out line (1) (and optionally add group = id to the aesthetics) then it does not generate an error.

Answer 1

For a discussion of this behaviour of ggplot2 see gihub. As the issue was closed and as far as I get it there is nothing you can do abozt that by using annotate. However, to achieve what you want you can simply use geom_rect like so:
library(tidyverse) # ggplot2_3.3.0
df_annotate <- data.frame(
xmin = seq(20, 50, 10),
xmax = seq(30, 60, 10),
ymin = -Inf,
ymax = Inf,
fill = paste0("grey", seq(60, 90, 10))
)
tibble(t = 1:100) %>%
crossing(id = LETTERS[1:4]) %>%
group_by(id) %>%
mutate(y = cumsum(rnorm(n()))) %>%
ggplot() +
geom_rect(aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax, fill = fill), data = df_annotate) +
geom_line(aes(t, y)) +
scale_fill_identity() +
facet_wrap(vars(id)) +
theme_light()
Created on 2020-05-28 by the reprex package (v0.3.0)
Edit Using ggnewscale it's possible to have a second or ... fill scale:
library(tidyverse) # ggplot2_3.3.0
library(ggnewscale)
df_annotate <- data.frame(
xmin = seq(20, 50, 10),
xmax = seq(30, 60, 10),
ymin = -Inf,
ymax = Inf,
fill = paste0("grey", seq(60, 90, 10))
)
df <- tibble(t = 1:100) %>%
crossing(id = LETTERS[1:4]) %>%
group_by(id) %>%
mutate(y = cumsum(rnorm(n())))
ggplot() +
geom_rect(aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax, fill = fill), data = df_annotate) +
scale_fill_identity() +
new_scale_fill() +
geom_area(data = df, aes(t, y, fill = id)) +
facet_wrap(vars(id)) +
theme_light()
Created on 2020-05-29 by the reprex package (v0.3.0)