Good news! Cemetech now has this too, in visual form, like their forums' daily post stats graph. Instead of monthly stats it shows daily ones with the moving average. For example you can see I had a huge period of absence after 2006, then most of my activity came around 2010: http://cemetech.net/forum/profile.php?mode=viewprofile&u=15 (although I typically logged in two weekends per month or on thursdays)

All graphs are generated during night. Just go to your profile there and have fun looking how your activity evolved over time. Note: If you signed up there several years ago, your graph might look kinda squished, although for some reasons I didn't realize I was the 15th member to ever sign up there!

EDIT: Note: like Omnimaga's current board, these graphs do not include stats from their previous forum database (on Invisionfree). I think my 2004-early 2005 activity was way higher than the two years after.