This is a two-line patch to tkcanvas.trm which allows
one to change the font used by gnuplot by setting the
global variable "gnuplot_font".
(The mechanism for using different fonts was already
built into the terminal but there was no clear way of
accessing it without doing a search/replace on
gnuplot's code; this way is easier.)
I'm new to all this, so do let me know if I've done it
wrong or overlooked something obvious.