# set terminal png truecolor nocrop font arial 8 size 420,320 
# 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 urange [ * : * ] noreverse nowriteback  # (currently [-5.00000:5.00000] )
set vrange [ * : * ] noreverse nowriteback  # (currently [-5.00000:5.00000] )
set zrange [ * : * ] noreverse nowriteback  # (currently [-100.000:100.000] )
set cbrange [ -1.00000 : 1.00000 ] noreverse nowriteback
set palette rgbformulae 31, -11, 32
set colorbox vertical origin screen 0.9, 0.2, 0 size screen 0.05, 0.6, 0 front bdefault
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 -3 notitle