switch to directly generating FIG output, bypassing gnuplot

[derekbruening]

Nearly all the bugs and breakage over time have involved gnuplot changing
how it does things or with gnuplot bugs. Maybe we could directly generate
FIG and avoid relying on gnuplot at all? The hard part is computing text
bounding boxes for placement of labels. In the past I looked into using
XTextExtents from perl X11::Lib or X11::Protocol but had trouble with
both.