How do I change the stacking order in a bar chart in ggplot2?

From the online bar chart guide:

qplot(factor(cyl), data=mtcars, geom="bar", fill=factor(gear)) 

How do I get 5 to sit on the bottom, 4 above that, and 3 on top?

qplot(factor(cyl), data=mtcars, geom="bar", fill=factor(gear), order = -gear)

qplot(factor(cyl), data=mtcars, geom='bar', fill=factor(gear, level=5:3))

To generalize @xiechao's solution (@hadley's doesn't work in latest ggplot), you can reverse the factor orders to achieve this:

library(ggplot2)
data(mtcars)
mtcars$gear <- factor(mtcars$gear)  # First make factor with default levels
mtcars$gear <- factor(mtcars$gear, levels=rev(levels(mtcars$gear)))
qplot(cyl, data=mtcars, geom="bar", fill=gear)
# Or with ggplot
ggplot(mtcars, aes(factor(cyl), fill=gear)) + geom_bar()