Feature request add strip.background.y and strip.background.x ?

[smouksassi]

With the switch argument and strip placement one might want to color the x and y facets differently e.g. below there is no way to keep the y facets gray while coloring the x ones white

ggplot(mtcars, aes(cyl,disp))+
facet_grid(vs~am,switch="x")+
theme(strip.placement="outside",
strip.background=element_rect(fill="gray"))

[hadley]

Pretend reprex:

df <- data.frame(x = 1:2, y = 1:2)
ggplot(df, aes(x, y)) +
  facet_grid(x ~ y) +
  theme(
    strip.background.x = element_rect(fill = "red"),
    strip.background.y = element_rect(fill = "green")
  )