Gaphor is a UML, SysML, RAAML, and C4 modeling application. It is
designed to be easy to use, while still being powerful. Gaphor implements
a fully-compliant UML 2 data model, so it is much more than a picture
drawing tool. You can use Gaphor to quickly visualize different aspects
of a system as well as create complete, highly complex models.

Gaphor provides four modeling languages: UML, SysML, RAAML and C4 and
makes them accessible to beginners.