geographic plotting

This question has been posed before (a few years back) but I'm 
hoping someone may be able to point me to the state-of-the-art,
if there is such.

I'd like to be able to produce via gnuplot a map of (for example) 
the USA showing the states colored by median income, or PM2.5 
particulate emissions, or whatever.

I understand that the starting point would be a suitable US states 
shapefile (I know where I can get that), which would have to be 
translated to a format (resembling world.dat, I suppose) that can be 
understood by gnuplot. I have some notion of how that might be done.

The next issue would be to get gnuplot to colorize specified areas 
of the map according to the values of some additional data. 
Unfortunately I have little notion of how to do that.

Please don't point me to R. I know this sort of thing can be done 
via R packages -- with the help of hundreds of megabytes of 
additional GIS software! But it seems to me it should be doable much 
more simply, perhaps using the shapelib library (a few hundred 
Kbytes in size), if I could just figure out how to do a heatmap-type 
thing but filling irregular polygons rather than just rectangles.

-- 
Allin Cottrell
Department of Economics
Wake Forest University