r - Controlling the 'alpha' level in a ggplot2 legend

Question

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.

Answer 1

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)))