Could you post here a diagram as example, so that we could better understand what you are trying to do.
About State Diagram, you have to know that Dot/Graphviz is used for the rendering, and sometimes it gives limitation on what we can do.
You can add the current line in your diagram:
!pragma svek_trace on
This will generate on your disk the temporary dot files used by PlantUML : it may help you to understand how the whole process works.