r - Controlling ggplot2 legend display order

Question

Does anyone know how I can get control of the ordering of legends in ggplot2?

From what I can see the order appears related to the actual scale labels rather than the scale declaration order. Changing the scale titles alters the ordering. I've made a small example using the diamond dataset to highlight this. I'm trying to use ggplot2 for a series of plots and I want to make one variable appear on the right in them all. At present though this only happens in some of them, and I'm at a loss on how to enforce my desired ordering whilst retaining the appropriate scale labels.

library(ggplot2)
diamond.data <- diamonds[sample(nrow(diamonds), 1000), ]
plot <- ggplot(diamond.data, aes(carat, price, colour = clarity, shape = cut)) +
  geom_point() + opts(legend.position = "top", legend.box = "horizontal")
plot # the legend will appear shape then colour 
plot + labs(colour = "A", shape = "B") # legend will be colour then shape
plot + labs(colour = "Clarity", shape = "Cut") # legend will be shape then colour

Answer 1

In 0.9.1, the rule for determining the order of the legends is secret and unpredictable. Now, in 0.9.2, dev version in github, you can use the parameter for setting the order of legend.

Here is the example:

plot <- ggplot(diamond.data, aes(carat, price, colour = clarity, shape = cut)) +
  geom_point() + opts(legend.position = "top")

plot + guides(colour = guide_legend(order = 1), 
              shape = guide_legend(order = 2))

plot + guides(colour = guide_legend(order = 2), 
              shape = guide_legend(order = 1))