COGRE is a COnnected GRaph Editor. You can use it to create simple connected graphs and diagrams. You can also create more complex UML class diagrams in it, or even bind UML diagrams directly to source code, navigating your source via the UML diagram.
While COGRE does have a lot of functionality, it still needs more work. Consider improving COGRE.
Read more a bout it in see the COGRE manual.