Here's an example from a Datadog dashboard: https://cloud.githubusercontent.com/assets/1189716/4064956/6...
I wish such a chart style was more common, it's so much more useful than a typical line chart of the median/p9x. Unfortunately, I'm not familiar enough with D3 and such to write my own; it would likely be crap.
http://bl.ocks.org/mbostock/4060606 http://www.trulia.com/trends/2011/09/house-hunter-by-day-not... http://prcweb.co.uk/circularheatchart/ http://bl.ocks.org/tjdecke/5558084
Heck if you point me to a stock dataset I could probably code up a simple example that you could tweak. (no promises though ;)
We also have the notion of "interactors" which allow you to add draggable widgets to a plot: http://ncnr.nist.gov/instruments/magik/calculators/calcR_wf....
To see the heatmap in action, you can look at: http://ncnr.nist.gov/ipeek/singleplotwindow.html?instrument=...
There are of course glitches, but hopefully you find some of this useful!
I think it's d3.scale.color to automatically set up a gradient you plug a value into and get a color out.