@femto - that's a good point. That occurred to me, but for the moment I decided to stick with the red/green that was already used for the previous visualization. Well sort of, as I actually use d3.rgb.darker on the actual colors because I wasn't satisfied with the original ones, as they looked too light. Even for red/green, I think there's a lot of room for improvement here to make them stand out more.

I'm not sure off the top of my head what would be best for the rg replacement. Maybe some subtle shape differences, etc.