This picture was produced by Mathematica using the command 
PlotSection[-x^2 - y^2,{x,-1,1},{y,-1,1},
	XSection->1/2,YSection->-3/4,ZSection->-1,VSection->x/2,
	OSection->-x + y/2 - 1/2,
	XSectionStyle->RGBColor[0,1,0],
	YSectionStyle->{RGBColor[1,0,1],RGBColor[0,0,1]},
	VSectionStyle->RGBColor[1,1,0],
	Axes->False,Boxed->False];
PlotSection and hundreds of other Mathematica were commands were developed by Dennis Schneider.