UML Diagrams Plugin
This Zim plugin allows you to insert a UML diagram in a page.
Go to the the plugin repo and follow instructions. You don't even need to clone it!
How does it work?
Go to Insert > UML Diagram. An editor window will open up, where you can type PlantUML script and preview the resulting diagram. Click OK and your diagram appears in the page.
You can modify a diagram with a right click on it > Edit.