# set terminal pngcairo  background "#ffffff" font "arial,10" fontscale 1.0 size 500, 300 
# set output 'hidden2.1.png'
set style fill   solid 0.50 border
unset key
set isosamples 25, 25
set hidden3d front offset 1 trianglepattern 3 undefined 1 altdiagonal bentover
set xyplane at 0
set title "Mixing pm3d surfaces with hidden-line plots" 
set cbrange [ -1.00000 : 1.00000 ] noreverse nowriteback
set palette rgbformulae 31, -11, 32
f(x,y) = sin(-sqrt((x+5)**2+(y-7)**2)*0.5)
GPFUN_f = "f(x,y) = sin(-sqrt((x+5)**2+(y-7)**2)*0.5)"
splot f(x,y) with pm3d, x*x-y*y with lines lt 1 lc rgb "#000000",       f(x,y) with lines lt -2 notitle