ggplot2 make legend key fill transparent

Question

I am trying to make the legend key fill for a ggplot transparent. I followed the instructions on one of Hadley\'s ggplot2 guides for changing the legend key fill, but for some

Answer 1

To make use of the transparency levels, one can use:

For the entire legend:

theme(legend.background=element_rect(fill = alpha("white", 0.5)))

alpha("white", 0) being completely transparent like element_blank(), and alpha("white", 1) having no transparency.

Now, for the key - if key and background have different transparencies:

theme(legend.background=element_rect(fill = alpha("white", 0)),
      legend.key=element_rect(fill = alpha("white", .5)))

Note: background transparency overrules the one for the key, i.e. background alpha must be smaller than key alpha.