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R: Using Alpha to Control Fill

I'm trying to use alpha to control the transparency of the fill of my graphic. I understand that the alpha function has been moved to the scales package, but I still can't get it to work.

Data Set:

Groups<-data.frame(Level=c("High", "Above Average", "Average", "Below Average", "Low"),       
Start=c(7.5, 5.5, 4.5, 2.5, .5), End=c(11.5, 7.5, 5.5, 4.5, 2.5)) 
d<-data.frame(x=1:10, y=1:10)
basicBox<-ggplot(d, aes(factor(x), y)) + geom_boxplot()
basicBox + geom_rect(aes(NULL, NULL, xmin=Start, xmax=End, fill=Level), ymin=0, ymax=20,       data=Groups) + scale_fill_manual(values=alpha(c("red", "blue", "green", "grey", "purple"), 0.2))

Similar code can be found on page 86 of Hadley Wickham's ggplot2. When I don't load the scales library, I get and error saying that R doesn't recognize the alpha function, but even after I load scales I can't get alpha to work properly.

Any help would be greatly appreciated.