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I've not used Cacti for very long, so perhaps this is a common thing people do that I've not seen, but I thought I'd let everyone else know anyhow.
Personally, I didn't like the default Traffic Graphs in Cacti. I just upgraded to version 0.8.7a, and found that I had the new opacity features, so I played around with them for quite some time, but just couldn't come up with two areas that looked decent when they overlapped. Finally, I came upon a graph on (of all things) a political website that looked great and gave me an idea...
Graph the same data twice - Once as a 100% opaque line, and again as an area with 10% opacity. This would (at least in theory) give a nice solid edge to the lightly colored area, and the areas would blend together fairly well. (I first did this on a different graph and used red and blue - Makes a nice purple!) The Green and Blue don't blend quite as nicely, but it still looks very good.