# set terminal pngcairo  background "#ffffff" enhanced font "arial,10" fontscale 1.0 size 600, 400 
# set output 'contours.4.png'
set key at screen 1, 0.9, 0 right top vertical Right noreverse enhanced autotitles nobox
set style textbox opaque margins  0.5,  0.5 noborder
set view 60, 30, 0.85, 1.1
set samples 20, 20
set isosamples 21, 21
set contour base
set cntrparam levels discrete -75,-50 ,0 
set style data lines
set title "discrete set of contours (at -75, -50, 0)" 
set xlabel "X axis" 
set ylabel "Y axis" 
set zlabel "Z " 
set zlabel  offset character 1, 0, 0 font "" textcolor lt -1 norotate
splot x*y