# set terminal pngcairo  background "#ffffff" enhanced font "arial,10" fontscale 1.0 size 600, 400 
# set output 'discrete.2.png'
set contour base
set cntrparam levels discrete 0,15 ,75 
set style data lines
set title "3 discrete contours at 0 15 & 75" 
splot x*y