Plot3d seems to do something horrible with scaling.
Look at the following simple plot.
This gives a stepped plot, with only 10 distinct z values.
Along the same lines,
gives a plot with only *2* distinct z-values.
I get division by zero.
Maxima version: 5.9.0
Maxima build date: 19:10 2/9/2003
host type: i686-pc-mingw32
lisp-implementation-type: Kyoto Common Lisp