As you can see from the logo, UMLGraph aim to let user draw UML without using mouse. It generate UML diagram (Class diagram, Sequence diagram) based on a set of declarative syntax.
The most eye catching feature of UMLGraph is its ability to generate Class diagram from Java source code and embed it into javadoc. If you are generating javadoc via Ant build script, you can intergrate its doclet into your Ant script.
And after all these is done, you will get a nice javadoc with the most up to date Class diagram, without any changes to your source code.