ggplot2 avoid boxes around legend symbols

Question

Consider the example plot below. I'd like to make the little boxes around each of the symbols in the legend go away. How an I do this?

 ggplot(mtcars, aes(wt, mpg, shape=factor(cyl))) + geom_point() + theme_bw()

Answer 1

You're looking for:

 + opts(legend.key = theme_blank())

You can see lots of examples of all sorts of this stuff in ?opts. I couldn't remember off the top of my head which one it was, so I just tried a few until I got it right.

Note: Since version 0.9.2 opts has been replaced by theme:

+ theme(legend.key = element_blank())