Actually not sure how to do this with magit, but I just tried `vc-region-history' while highlighting an R method signature and body and got a pretty slick view of all the relevant commits. Would definitely prefer magit for jumping to commits, might take a few lines of elisp.

EDIT: Actually, [0] mentions =L, which seems to work flawlessly. Hacking on an option to take the selected region instead of manually specifying file and lines is easy. Alternatively, magit-blame lets you look at a specific change's commit.

[0] https://github.com/magit/magit/issues/1717