I added contour lines (map here) to my IDW map (read here). The map has contour breaks at 25, 50, and 75. I was really flying by the seat of my pants coming up with this algorithm – it’s pretty computationally intensive, but it does run (at least on Firefox), so don’t panic if the “This script is non-responsive” pop-up pops up. If there’s any demand for it, I’ll write up an explanation of what’s going on, but until then you’ll have to figure it out from the source code (good luck). And, if you’ve got a more streamlined raster to contour routine, I’d love to hear about it!