[racket] Controlling the size of plot symbols

Neil Toronto writes:

 > On git HEAD this takes about 15-20 seconds on my computer, depending on 
 > random sphere placement and size. It takes 5-6 seconds for 81 samples 
 > instead of 121. For comparison, the following takes 7-10 seconds with 
 > 121 samples and 3-4 seconds with 81 samples.

Thanks, I'll try that approach next week. I wouldn't use a
sphere-based representation for more than about 100 atoms anyway, and
I can probably reduce the number of samples even further. 30 samples
looks good enough for interactive use.

Is there any particular reason for choosing 81 or 121 samples?
Are those magic numbers?

Konrad.