linear gray colormap
cmap=graycolormap(n)
:n integer >= 1, the colormap size. : :cmap matrix with 3 columns [R,G,B]. :
graycolormap computes a colormap with n gray colors varying linearly from black to white.
f = `scf`_();
`plot3d1`_();
f.color_map = graycolormap(32);