The diagrams are written inside Tiddlywiki tiddlers, using the PlantUML syntax, and will be shown as images when the tiddler is opened.
It only works online, as it uses the online PlantUML server to generate the images.
To use it, you'll have to use the macro
plantUml in your tiddler, with the following format:
<<plantUml 'diagram specification' 'alternate text'>>
'alternate text' is optional.
Feel free to address your comments/suggestions/whatever to yours truly:
Aníbal Cáceres firstname.lastname@example.org