r - Legend placement, ggplot, relative to plotting region

Question

Problem here is a bit obvious I think. I'd like the legend placed (locked) in the top left hand corner of the 'plotting region'. Using c(0.1,0.13) etc is not an option for a number of reasons.

Is there a way to change the reference point for the co-ordinates so they are relative to the plotting region?

mtcars$cyl <- factor(mtcars$cyl, labels=c("four","six","eight"))
ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
opts(legend.position = c(0, 1), title="Legend placement makes me sad")

Cheers

Answer 1

Update: opts has been deprecated. Please use theme instead, as described in this answer.

Just to expand on kohske's answer, so it's bit more comprehensive for the next person to stumble upon it.

mtcars$cyl <- factor(mtcars$cyl, labels=c("four","six","eight"))
library(gridExtra)

a <- ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
opts(legend.justification = c(0, 1), legend.position = c(0, 1), title="Legend is top left")
b <- ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
opts(legend.justification = c(1, 0), legend.position = c(1, 0), title="Legend is bottom right")
c <- ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
opts(legend.justification = c(0, 0), legend.position = c(0, 0), title="Legend is bottom left")
d <- ggplot(mtcars, aes(x=wt, y=mpg, colour=cyl)) + geom_point(aes(colour=cyl)) + 
opts(legend.justification = c(1, 1), legend.position = c(1, 1), title="Legend is top right")

grid.arrange(a,b,c,d)