This add on adds a heatmap of your card intervals in the statistics window.
This is the first add on I have written, so please report any bugs on GitHub: https://github.com/ChrisK91/CardHeatmap/issues Source code is available over there as well.
You can change the size of the heat map squares by editing /cardheatmap/cardheatmap.py, and adjusting maxCountX to your liking.
2018-02-10: Initial release
This add on is licensed under the AGPLv3 license, and redistributes d3.js in version 4.13.0 by Mike Bostock, licensed under the BSD license
This add on is very similar (although not as complex as) Review Heatmap by Glutanimate. I took his code as a starting point.