@WeatherGod: You are correct, I have not explained the fix in the code but pretty much nothing is explained in the existing mplot3d code.  As I keep saying, my changes are consistent with the existing code.

The more I think about it the less I want my name associated with the mplot3d codebase, hence I am closing this PR.  I have diagnosed the problem and come up with a fix, albeit possibly a poor fix, so you or someone else can use my code as the basis for an acceptable fix if you wish.

You still owe me a pint as for figuring about the bug as you promised in #1299!