After a user clicks on the streamgraph, the bottom-left displays information about commits by the corresponding contributor in the corresponding quarter; the contributor name is already a link to their GitHub profiles, but the "changes" count could also be made a link to the actual commits.

These links would have the format:

https://github.com/git/git/commits?author=gitster&since=2005...

or

https://github.com/git/git/commits?author=gitster@pobox.com&...