Each page should include a concise definition of the object or process being visualized, preferably with a link to some online resource explaining the topic in detail.
The approach to visualization should be clearly defined, with examples. The page should include links to other approaches to visualizing the same thing, and (if appropriate) links to examples of visualizing other things in the same way. If there are several variants of the approach, they can be listed on one page or several inter-linked pages.
A bit of context or history is also nice --- when did you first see (or use) this diagram in a lecture or book? Is it primarily used in discussion of specialized research or to teach some basic principle?