Create a custom measure, the number of unique authors, in Git Commit Log

Hi,

In demo dashboard, Git Commit Log Analysis - RoR, just wondering how to create a calculated measure for the number of unique authors?

That is, except the default Commits, Additions, Deletions, Changes, …, we would also like to know the number of unique authors during that time period. Something this:

datetime, commit, unique authors
Q1 2019, 1440, 30
Q2 2019, 1560, 37

Best,
Drake