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This may have been what you're thinking of. Sadly I lack a source; it's just been kicking around my miscellaneous images folder.

Thank you! I love the internet :)

That's it! Thanks!

So where would categories hang on that graph? Below semigroups?

…which is reachable from the more memorable http://www.math-atlas.org/

There is one in the book, Mathematics: Form and Function by Saunders Mac Lane (who was one of the creators of category theory).