Request: multiple colours for facet strip.background

[alexconingham]

This code generates the plot, and changes the strip backgrounds to red:

p <- ggplot(mpg, aes(displ, cty)) + geom_point() + facet_grid(. ~ cyl) +
theme(strip.background = element_rect(fill="red"))

It would be useful to be able to have each strip a different colour, something like the below seemed intuitive though just makes them red (first colour in the call is used)

p <- ggplot(mpg, aes(displ, cty)) + geom_point() + facet_grid(. ~ cyl) +
theme(strip.background = element_rect(fill=c("red","green","blue","yellow")))

[stefanedwards]

AFAIK it is entirely doable, but introducing vectorized theme elements as you propose raises the question of what users will expect when they are applied to other aspects.

In the meanwhile, here is a solution that modifies the resulting grob. The plot is one of the first examples from facet_grid.

p <- ggplot(mpg, aes(displ, cty)) + geom_point() + facet_grid(drv ~ cyl)

g <- ggplot_gtable(ggplot_build(p))
stripr <- which(grepl('strip-r', g$layout$name))
fills <- c("red","green","blue","yellow")
k <- 1
for (i in stripr) {
  j <- which(grepl('rect', g$grobs[[i]]$grobs[[1]]$childrenOrder))
  g$grobs[[i]]$grobs[[1]]$children[[j]]$gp$fill <- fills[k]
  k <- k+1
}
grid.draw(g)

Might be pretty with some other colour choices...

[dickie-roper]

Sorry, I'm not particularly great in R, but would it be possible to map the fill of the facet strip background to the variable that is being used for the facet (or some other variable)?

Something like...
... + facet_grid(. ~ cyl, aes(fill=cyl))

I know it may seem a bit redundant to also colour by the facetting variable but there are cases where it can add some clarity.