Source: https://github.com/pyrochlore/obsidian-tracker
The image above comes from a plug-in for Obsidian.
The following picture is from a custom QCalendarWidget I made that shows a different color depending on the % of tasks completed for each day.
(https://i.stack.imgur.com/unmMu.png)
I've found this answer from @eyllanesc showing how to make a custom QCalendarWidget that displays a round edge around the selected day. I wonder if it's also possible to make it looks like the picture I posted above.