Controlling the 'alpha' level in a ggplot2 legend

ggplot2, r

Solution

You can control semitransparency with function `alpha()` from package `scales` by providing color and alpha value. This function can be used inside `element_rect()` when you provide color for `fill=`.

library(scales)    
p<-ggplot(iris,aes(Petal.Length,Petal.Width,color=Species))+geom_point()
p+theme(legend.position=c(1,1),legend.justification=c(1,1),
        legend.direction="vertical",
        legend.box="horizontal",
        legend.box.just = c("top"), 
        legend.background = element_rect(fill=alpha('blue', 0.4)))

Problem

In ggplot2, how can I make the legend have a semi-transparent background. The following code, gives a fully transparent background (and positioning control) ``` plot <- plot + theme(legend.position=c(1,1),legend.justification=c(1,1), legend.direction="vertical", legend.box="horizontal", legend.box.just = c("top"), legend.background = element_rect(fill="transparent")) ``` But how can one control the level of alpha, I don't believe that `element_rect` has that ability.

Original source