math.SG is a real category (or not)

I’ve been playing around with data I collected from arXiv, and more particularly metadata of preprints submitted in or cross-listed with math.SG (see here for more).

Below is the diagram obtained by drawing a dot with coordinates $(x,y)$ for each category cat where:

• $x$ is the number of preprints submitted in cat which has math.SG as secondary category,
• $y$ is the number of preprints submitted in math.SG which has cat as secondary category.