Create and maintain UML diagrams in your Google Docs™ and Google Slides™. The diagrams are drawn by PlantUML, a text-based UML diagramming tool, and inserted into your Google Docs as images. You don't need to spend time with the layout of your diagram; PlantUML takes care of that thanks to Graphviz algorithms. 

The PlantUML source text is encoded within the link of the image in Google Docs™ or Google Slides™, so you can recover the source text and edit it later, re-inserting the diagram using PlantUML Gizmo.

There are some sample diagrams in PlantUML Gizmo to help you get started.