To follow up: The problem is when you change the colormap and not easily solved in the freezeColors / unfreezeColors. I have created a simple fixme to get round it which I can provide if anyone needs it.
Dang, you are right of course. In my excitement to see something that would plot geographic-coordinate vectors I failed to internalize the very clear statement that the code assumes a gridded field. Guess I'll keep looking and/or work on writing some code to do what I need.
Thanks for your prompt response. It's much appreciated.