Yeah, that's one of the papers I saw that left me confused. The whole paper is about sgplot so I still don't know if I can use transparency in gplot, and I don't know what "primitives" are. I know sgplot is nice for putting several plots together, but I prefer to do that using annotate to manually add whatever I want since I don't like the way sgplot outputs the image files. Maybe I just need to learn how to output image files better...