backgrounds.R

df <- data.frame(
  x = 1:5,
  y = 1:5,
  z = factor(c(1, 2, 3, 3, 3))
)
ggplot(df, aes(x, y)) + 
  geom_rect(aes(ymin = -Inf, ymax = Inf, xmin = x - 0.5, xmax = x + 0.5, fill = z)) +
  geom_point() + 
  scale_fill_grey()
  

This seems to work fine for geom_bar()

ggplot(df, aes(x, y)) +
  geom_rect(aes(ymin = -Inf, ymax = Inf, xmin = x - 0.5, xmax = x + 0.5, fill = z)) +
  geom_bar(stat = "identity") + 
  scale_fill_grey(start = .9, end = .6)