How do I reverse the Y axis in ggplot's geom_tile?

Question

Below is a graph. How do I reverse the Y axis , so it reads down "a b" instead of "b a", while leaving the X axis alone?

Code:

library(ggplot2)
levels = ordered(c('a', 'b'))
data = data.frame(x=ordered(c('a', 'a', 'b', 'b'), levels=levels),
                  y=ordered(c('a', 'b', 'a', 'b'), levels=levels),
                  prob=c(0.3, 0.7, 0.4, 0.6))
ggplot(data, aes(x, y)) + geom_tile(aes(fill=prob))

Answer 1

I figured this out.

ggplot(data, aes(x, ordered(y, levels=rev(levels)))) + 
  geom_tile(aes(fill=prob))