Hi,

How can I plot two boxes with gnuplot in 3 Dimensions, one small box inside one big box? 
I know that it is possible with 
plot3d(Polygon([[x11, y11], ...[xln, yln]], [[x21, y21], ...[x2n, y2n]], ...[[xm1, ym1], ...[xmn, ymn]])

but how can I do with mathematical equations?