I am the lead software engineer of the AdvoCATE toolkit, developed at NASA Ames Research Center.
AdvoCATE is the state of the art in assurance case development and aims to be a one-stop shop developing, maintaining, and understanding assurance cases.
It’s written in Java as an Eclipse product and leverages Sirius, NatTable, EMF, and many other facets of the Eclipse Modeling Framework.
A few screenshots of the diagrams created in AdvoCATE are shown below:
We are working on making AdvoCATE available to the outside world, but it is not quite there yet.