# set terminal pngcairo  background "#ffffff" enhanced font "arial,10" fontscale 1.0 size 600, 400 
# set output 'discrete.3.png'
set contour base
set cntrparam levels incremental -20,5,10
set style data lines
set title "9 incremental contours starting at -20, stepping by 5" 
splot x*y