@John Blackbourn -
There’s an error though (looking at rev 3). The is_tax() conditional isn’t true for category and tag archives as the diagram states.

Oh, dear! Thanks for pointing that out. I inferred that from the is_tax() Codex page, which just states "Taxonomy" as opposed to custom taxonomy, and which also lists all of the core-registered taxonomies.

(Note to self: update is_tax() Codex page to clarify.)

The “Taxonomy Archive” block should be removed, and the “Category Archive”, “Tag Archive”, and “Custom Taxonomy Archive” blocks should be moved up to the same level as “Date Archive”, “Author Archive” and “Custom Post Type Archive”.

Sure enough! Fixed in my working copy.