A bit of searching later found the right incantation to make gnuplot adjust
color based on a third value (signal level in my case). It isn't very complicated:
set size square
set angles degrees
set grid polar 30
set xtics axis 0,30
set ytics axis 0,30
set title "GPS satellite tracks"
set xlabel "Azimuth"
set ylabel "Elevation"
set terminal png size 600,600
set output "gpsazelsig.png"
plot "gpsazelsig.dat" using 1:2:3 palette notitle
But the resulting plot
isn't very helpful for my original question
: in which direction radio
signals are obstructed. There are some obstructions in the Southwest, but they
are comparable to what is in the Northeast.