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. 

IMPORTANT: The PlantUML source text is encoded within the link (hyperlink) of the image in Google Docs™ or Google Slides™. PlantUML Gizmo recovers the source text allowing you to edit it later. DO NOT REMOVE THE LINK (hyperlink) on the PlantUML image if you want to keep its source text. Sadly, in Google Docs™, changing the "Text wrapping" of the image to anything but "Inline with text" will remove the link. Unfortunately, you won't be notified by Google's application that it does this. 

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