I’d like to propose to make PlantUML our best practice format when adding diagrams on xwiki.org (rather than insert images for which we don’t have the sources).
Manuel mentioned a few that provide live rendering (or close to live, with the hit of a button). It’s probably nicer than visual editors to learn the syntax.
Note that the learning was not an issue for me when I used it for the first time to convert the 2 existing diagrams. I was really amazed at how fast I was able to do it (I was expecting to struggle a lot more ;)).