Thought: # of remaining virgin pixels is a little clouded by a couple things: people making mistakes then whiting them out, and people using default white space as part of designs
(not sure what you mean by # surviving pixels, but this was my first thought)
I'd like to see an overall heat map, yeah. I know that area in the top left quadrant with the four-color paint roller used to be something else that a user and their multiple alts tried hard to get on the Canvas.
Embarrassingly, I'm on here.