Files 'extern.h' and 'plotter.h' are based/taken from GNU plot-utilities
(plotutils-2.3) by 

Robert Maier <rsm@math.arizona.edu> wrote the libplot/libplotter library
and the graphics programs graph, plot, tek2plot, and plotfont, beginning in
1995.  The old `GNU graphics' package (written by Rich Murphey
<rich@freebsd.org> in 1989-1991) served as a starting point, but the
plotutils package is an order of magnitude larger.  Probably none of the
code from the earlier package remains in libplot/libplotter or the graphics
programs.                                                                       

