# set terminal pngcairo  background "#ffffff" enhanced font "arial,10" fontscale 1.0 size 600, 400 
# set output 'pm3d.3.png'
set border 4095 front linetype -1 linewidth 1.000
set samples 25, 25
set isosamples 20, 20
set style data lines
set ticslevel 0
set title "pm3d at b (bottom)" 
set xlabel "x" 
set xrange [ -15.0000 : 15.0000 ] noreverse nowriteback
set ylabel "y" 
set yrange [ -15.0000 : 15.0000 ] noreverse nowriteback
set zrange [ -0.250000 : 1.00000 ] noreverse nowriteback
set cblabel "colour gradient" 
set pm3d implicit at b
splot sin(sqrt(x**2+y**2))/sqrt(x**2+y**2)