Order discrete x scale by frequency/value

Question

I am making a dodged bar chart using ggplot with discrete x scale, the x axis are now arranged in alphabetical order, but I need to rearrange it so that it is ordered by the

Answer 1

You can use reorder:

qplot(reorder(factor(cyl),factor(cyl),length),data=mtcars,geom="bar")

Edit:

To have the tallest bar at the left, you have to use a bit of a kludge:

qplot(reorder(factor(cyl),factor(cyl),function(x) length(x)*-1),
   data=mtcars,geom="bar")

I would expect this to also have negative heights, but it doesn't, so it works!