PyXPlot

set xlabel "$x$"
set ylabel "$y$"
set zlabel "$x^3/20+y^2$"
set nokey
set size 8 square
set nogrid
plot 3d x**3/20+y**2 with surface col black fillc blue, \
        x**3/20+y**2 with contours col black

Notes

A surface with contours projected beneath.