I turned this in as a final for art-history-of-games-ish class this semester, so I thought I'd put it up here for shits and giggles as well.
This is a somewhat-exhaustive taxonomy of PbtA games, tracking ten different systemic mechanical elements in each, and a loose kind of succession between them. It's not perfect, in that my methodology is sort of flawed and there are definitely errors and omissions floating around, but it's a useful visual nonetheless, I think.
Here's the PDF. You'll want Acrobat for this, since it's dependent on the different layers. If you don't have Acrobat or otherwise don't have access to the layers feature, here's a .zip file of all of the graphs as images.
And just to show the mild absurdity of this whole thing, here's what the graph looks like with every single layer turned on simultaneously:
Anyway, let me know if you have any questions or want to give me grant money to do more of this research or whatever.