The Hertzsprung–Russell diagram is a scatter plot of stars by temperature and luminosity.
Traditionally, it is shown with temperature axis pointing from right to left, which I don’t really like.
Stack Exchange family of sites is not (yet) as numerous as stars in the Universe; there are only 120 or so of them. But they can also be organized on a two-parameter log-log scatterplot. The two parameters are: total number of questions (intrinsic characteristic, like surface temperature) and the number of daily visits (luminosity in Internet terms).
The linear scale chart was not going to work, due to the supergiant size of Stack Overflow:
Even on the log-log scale Stack Overflow is an outlier, but within reason:
The colors follow Stack Exchange classification: Technology, Science, Culture & Recreation, Life & Arts, Business, Professional. The largest of science sites, and the second largest overall, is Mathematics, although it trails several non-science sites in luminosity.
Annotated version of the above diagram:
Both Mathematics and MathOverflow have low traffic compared to their size: perhaps the Internet audience is just not that into math? On the other hand, the young Mathematics Educators site is way up in the traffic category.
Ironically, Astronomy is a low-luminosity site, trailing in traffic the much smaller Earth Science. Other annotated science sites are well established by now: Physics, Statistics, and (slow-moving) Theoretical Computer Science.