RFC: define new variables TERM_XMIN etc.

[Allin C. <cot...@wf...>]

I think my original posting on this got lost in a flood of other 
things, so I'm trying again.

The idea: make available, via the print command, the "bounding 
box" of the actual plot area following a plot command.  This is 
primarily intended for retrieving the pixel bounds for bitmapped 
graphics so as to be able to construct an "image map" (e.g. for a 
gnuplot-generated PNG file), but it may have other uses.

Method: define 4 new internal variables, TERM_XMIN, TERM_XMAX, 
TERM_YMIN and TERM_YMAX.  These get updated in eval.c after a plot 
(along with GRAPH_X_MIN and friends).  A function is added to the 
term structure which enables -- but does not require -- a given 
terminal to supply a scale factor for these bounds.  This is 
designed for pngcairo, which works in units of pixel/20, but might 
have application for some other terminals.

Comment: This would be *very* useful for me, and others have 
expressed an interest in this sort of thing before.  In the 
implementation I have tried to take into account suggestions 
from Hans and Ethan, and to respect the gnuplot architecture.

Patch: The required small diffs to cairo.trm, eval.c and 
term_api.h (as per current CVS) are attached in a tarball.

-- 
Allin Cottrell
Department of Economics
Wake Forest University, NC