15.64.50  SHADOW 

This causes a 3-d plot to be shadowed onto the back side where the grid is
drawn.
SET SHADOW [ALL[=ON|OFF]] [SYMBOL[=ON|OFF]] [Y|X!Z[=ON|OFF]] [XY|YZ|
          ZX[=ON|OFF]]
     [INTENSITY|WIDTH=n]
     [NOCOLOR|WHITE|RED|GREEN|BLUE|YELLOW|MAGENTA|CYAN]
     [NOTEXTURE|SOLID|DOTS|DASHES|DAASHES|DOTDASH|PATTERNED|FUNNY|SPACE]
     [PERMANENT]

To produce shadowing you must select at least one plane to shadow the plot
in (XY,YZ,ZX).