ggplot2: plotting order of factors within a geom

Question

I have a (dense) dataset that consist of 5 groups, so my data.frame looks something like x,y,group. I can plot this data and colour the points based on their group using:

p= ggplot(dataset, aes(x,y))
p = p + geom_point(aes(colour = group))

My problem is now only that I want to control which group is on top. At the moment it looks like this is randomly decided for (at least I don't seem to be able to figure out what makes something be the "top" dot). Is there any way in ggplot2 to tell geom_point what the order of dots should be?

Answer 1

The order aesthetic is probably what you want.

library(ggplot2)
d <- ggplot(diamonds, aes(carat, price, colour = cut))
d + geom_point()
dev.new()
d + geom_point(aes(order = sample(seq_along(carat))))

The documentation is at ?aes_group_order

Answer 2

When you create the factor variable, you can influence the ordering using the levels parameter

f = factor(c('one', 'two'), levels = c('one', 'two'))
dataset = data.frame(x=1:2, y=1:2, group=f)
p = ggplot(dataset, aes(x,y))
p = p + geom_point(aes(colour = group))

Now, ggplot uses this order for the legend.