[sunspeck]

Have you considered a chloropleth[1]? Similar effect to a heat map but likely much easier to implement.

Just shade each of your already defined regions by [sightings/area].

[1] http://en.wikipedia.org/wiki/Choropleth_map

[meredithmmyers]

I hadn't for this project - to be honest I didn't have a clear plan for this project, it was just something I threw together in a few hours for fun because I thought the dataset was amusing :) A chloropleth is a great idea though. If I find the time to improve this, I'll definitely keep that in mind!