One thing to note is that the Chromium and Mozilla repos host browsers as well as engines, whereas the WebKit and Servo repos host engines only. (The most prominent browser front-end projects are Safari, desktop and mobile, and browser.html respectively, which are hosted in separate repositories.)
Wow, I did not expect that Servo would be comparable to Webkit in number of unique authors per month, as young as it is. I know the Servo devs put a great deal of effort into outreach and making it easy for new contributors to jump in, and that appears to have paid off. :)<p>EDIT: Actually, now I'm curious about the methodology of how the numbers are collected. I know that Servo is designed to consistent of independent components contained in separate repositories whereas IIRC mozilla-central is fairly monolithic, so if you look only at the repo at <a href="https://github.com/servo/servo" rel="nofollow">https://github.com/servo/servo</a> you may be underestimating the the activity on the project.
It'd be interesting to see similar stats for less well-known browsers like NetSurf and Dillo, which are also far simpler. I doubt they have anywhere near as many developers as the big ones, but as the saying goes, "quantity is not quality"...
Thank you to the author for making the colours easily distinguishable for people with colour vision deficiencies in most of the graphs. Graphs like these tend to be spectacularly bad at this, putting aesthetics over accessibility.
I find it interesting that, in the third graph (commits/dev), the Mozilla line gets a dip to about half its previous value as the Chromium line shows up, somewhere around mid-2008. Post-dip the jitter also seems to decrease.