set terminal png size 840,720
set encoding utf8
set style arrow 1 nohead
set xlabel "x"
set ylabel "z"
set size ratio 1
unset key
set output "plot.png"
rgb(r,g,b) = int(r)*65536 + int(g)*256 + int(b)
colorof(x) = (x + 10) * 192 / 20
shade(x) = rgb(x,x,x)
set object 1 rect from -5.5,26.9 to 5.5,27.1 fc rgb "black" front
set label 10 at -6,26.75 "Σ" font "arial,30" front center
plot 'rect_coords' using 2:3:4:5:(shade(colorof($1))) with vectors lc rgb variable

