For the Scherk2 surface, the equation of a hyperbola in quadrant 1, with vertex at radius of 1 unit, is x*y = 1.
Points (x,y) which produce a z value are contained within the 4 hyperbolic curves.
(In contrast, the Scherk1 surface has a rectangular x,y source, so it was easier to program.)

I think that FXGraph3D could use a numpoints input(?)