Top key missing in guide_bins when reverse = TRUE

[petrbouchal]

When guide_bins() is used with the reverse param set to TRUE, the key for the top bin is missing (and it seems the entire scale is shifted down in the key).

Tested on current dev ggplot2.

library(ggplot2)
df <- data.frame(a = 1:100, b = 1:100)

ggplot(df) +
  geom_point(aes(colour = b, x = a, y = b)) +
  guides(colour = guide_bins(reverse = T))

[petrbouchal]

When filing #4234, I realised it is not clear that the default direction of the guide makes sense with large numbers at the bottom.