TagHistory is a tool letting you plot graphs showing tag usage stats over time.
You can compare multiple tags, and so use it to observe a shift in usage over time from one tag to another
Martin Raifer's diary entry gives more details
Math1985' diary entry gives more example tag comparisons (and their histories)
It has an API! See TagHistory API docs
Developed by Martin Raifer.
Source code and bug tracking on github: https://github.com/tyrasd/taghistory