# Date: 2025-08-14
# Please use or revise this code without any copyright concern. Thanks.

set term svg size 700,560 enhanced font 'Times,12'
set output 'max_paraboloid.svg'

set multiplot

set xlabel "x"
set ylabel "y"
set zlabel "z"
set xrange [-3.0:3.0]
set yrange [-3.0:3.0]
set zrange [-15.0:5.0]
unset colorbox
set view 60,15
set style fill solid 0.30 border

set cntrparam levels auto 20
set contour base
set isosamples 20,20
set samples 20,20
set pm3d
set hidd
unset key
set ticslevel 0

# Draw the surface of f(x,y) = 4-(x*x+y*y)
set palette defined (-15 "navy", 4 "cyan")
splot 4-(x*x+y*y)

# One label at the maxima.
set label 1 "{/=26 (0,0,4)}" at 0, 0, 6

# Two points.
set pointsize 1
unset pm3d
unset contour
splot 'twopts.txt' w p pt 7 lc "blue"
# Prepare a text file 'twopts.txt' as follows (no #s):
# 0 0 4
# 0 0 -15

unset multiplot
set term windows
set output