Interestingly, when I was doing the shading for the graphs, the same idea crossed my mind - all it requires is custom shading parameters. Currently they use just two when shaded - the colour and alpha at the bottom and the colour and alpha at the top (the colours are computed from the base colour set), but it wouldn't be too difficult to add more . Of course, you wouldn't (by default) get clearly delineated colours, more of a blend as the colours change. The other problem is it assumes a normalised vertical scale (0 to 1) - what you'd need here is the ability to specify absolute values for each colour as well.
So yes, do-able, UI is a bit of a challenge though. I've added it to the requests list.