ggplot with custom legend.text
===

I'm using ggplot to generate some graphics but get the following error:

    Error in FUN(X[[1L]], ...) : could not find function "label.theme"

when trying to specify the label names.

This basic graph gets generated correctly:

    ggplot(as.data.frame(list(a=1:5, b=1:5,c=factor(c(1,1,2,2,2)))), aes(x=a,y=b, color=c)) + geom_path(size=1)

But this graph produces the above error.

    ggplot(as.data.frame(list(a=1:5, b=1:5,c=factor(c(1,1,2,2,2)))), aes(x=a,y=b, color=c)) + geom_path(size=1) + opts(legend.text=letters[1:2])

Am I not setting the legend text correctly?