Raymond Toy - 2013-05-27

For the record, the offending command is

  [gnuplot_preamble, "set pm3d map"],

And, as expected, the problem is from sqrt(sin(theta)) when theta = 2*%pi.

It's easy enough to make plot3d honor the plot_realpart option, but plot_realpart defaults to false, so we still get the problem where NIL is encountered. I don't know what the right thing would be here.