I used the git repo at https://git.phial.org/d6/xz-analysis-mirror/commits/branch/m... and simply did

  git log --format=%cd --author="Jia Tan" 

and then processed it a bit with gnuplot. Should not be difficult to reproduce this graph, but I am not too much of a gnuplot wizard so I first preprocessed this into some different files in a REPL. Don't have the full code of what I did but it should not be difficult to reproduce, just parse the dates and look at the hours.