A few days ago I recall finding a visual calendar of my sign-in history on Math Stack Exchange. It looked like an ordinary monthly calendar, where the days were white if I hadn't signed in, and green if I had. (I found it when I wondering how close I've been in the past to getting the "fanatic" badge.)
However, I can no longer find this page/pop-up.
I thought it might have been a feature of Stack Exchange as a whole, but I was unable to find an answer on Meta Stack Exchange.