[Bug-plotutils] FAQ tips

[jidanni]

Please add to some plotutils FAQ:
Q: Can one plot to a dumb terminal using ASCII?
A: apparently no.

In Info at:
 display can be set by using the `--bitmap-size' option, or by setting
 the `BITMAPSIZE' environment variable.  For `graph -T tek', the
 graphics display is a square region occupying the central part of a

Mention that e.g., 400x333 is the format for --bitmap-size... else we
will try inches.