You've asked for it ;)

This prints the years - you can the group and plot them as you wish (it should be fairly easy, but I wrestled enough with git). It's a non-rigorous script (eg. it assumes nobody's email/name includes an `YYYY-MM-DD`-like string, and that filenames don't include the colon character):

    grep -P '\bTODO\b' -n -R -- * | awk -F: '{ system("git blame "$1" -L "$2","$2) }' | perl -lne 'print /(\d{4})-\d\d-\d\d/'

The working is actually fairly simple:

- grep prints filenames and lines

- awk captures the filename and line, and executes a git blame on it

- perl matches the year and prints it

In the Perl matching expression, month and day are not strictly necessary, but disambiguate potential 4-digit numbers in the email/name.

I'm very underwhelmed by the lack of customization of the `git blame` command - `--porcelain` is also uncustomizable, which makes things even uglier.

Note that `git blame` also mishandles some edges (printing "fatal: file [...] has only 1 line").