[racket] Controlling the size of plot symbols

Neil Toronto writes:

 > It's unfortunately not. But I think there's something you can do 
 > instead: use `parametric-interval' to draw the circles instead.

Thanks for the example, which contains some nice tricks I'll keep in
mind for some other application. But I don't see a way to use this for
my molecules, because I am need 3D plots. What I really want is
spheres.  If I draw my own parametric circles in 3D, I'd have to take
into account the view point and compute projections.

I did play with the idea of defining spheres as 3D surfaces, but I'd
expect this to be much too slow.

Konrad