[gnuplot] fillcurves

http://www.gnuplotting.org/tag/filledcurves/

http://gnuplot.sourceforge.net/docs_4.2/node245.html

   [closed | {above | below} {x1 | x2 | y1 | y2}[=<a>] | xy=<x>,<y>]

   filledcurves closed   ... just filled closed curve,
   filledcurves x1       ... x1 axis,
   filledcurves x2       ... x2 axis, etc for y1 and y2 axes,
   filledcurves y1=0     ... line y=0 (at y1 axis) ie parallel to x1 axis,
   filledcurves y2=42    ... line y=42 (at y2 axis) ie parallel to x2, etc,
   filledcurves xy=10,20 ... point 10,20 of x1,y1 axes (arc-like shape).

The above and below options apply both to commands of the form 

   ... filledcurves above {x1|x2|y1|y2}=<val>

and to commands of the form 

   ... using 1:2:3 with filledcurves below

[My Example]

f(x) = -0.75+2*x

g(x) = f(x)>0.5 ? 1./0:f(x)

h(x) = x>0.5 ? 1./0:x

set yr[0:]

p[0:1]  x-1  lc rgb "gold"  lw 1 w filledcurves y1=0.5 t '', g(x) lc rgb "forest-green" lw 1 w filledcurves y1=0.5 t '',  h(x) lc rgb "red"  lw 1 w filledcurves y1=0.5 t ''