Great points, but in the author's 2D histograms the use of 'color' to show point density is a little misleading. Color is not a scalar, it's a vector, and simply varying hue doesn't map out well to the human perception of lightness or intensity. It also doesn't allow for meaningful interpolation. If you take a look at http://vis4.net/blog/posts/avoid-equidistant-hsv-colors/ you can see a strategy for building perception-aware color palettes/gradients, which would be much more useful in this case.

(actually it looks like this wasn't the author's decision, but something built into the graphing library)