The advantage of an HTML heat map is that it can be made sortable. I've been happily using Stuart Langridge's sorttable.js code but I didnt think I could use it on the heat map because the cell values are spaces.
However reading the code, I see that Stuart supports sorting on an attribute sortable_customkey, so I could enable sorting of the heatmap by:
The result is a better visualization. I applied it to the scores computed in my readability analysis done earlier this week. Sorting the map helps me see much more clearly that the Fog and Flesch-Kinaid scores are positively correlated, which Flesch alone is negatively correlated with these scores. I would not have seen that in the static heatmap.
Thanks again @sil