[R] alpha parameter in function rgb to specify color

[Michael Toews]

Part of the problem is that alpha is a new and undocumented feature 
(note to developers: add this info into the pdf and rgb documentation). 
It only works in PDFs (maybe on quartz on Macs?), so you need to write 
to a PDF file:

pdf("out.pdf",version="1.4")
plot(1:10,col=rgb(1,0,0,alpha=seq(0,1,length.out=10)))
dev.off()

Now open up the PDF file...