Building a modified Call Hierarchy pane plugin. Looking for source code or examples to work with. Any pointers?

I'd like to extend the Call Hierarchy pane with features to provide more focus and context, as a primary navigation tool for API services and consumers, whose code is very much based around the branching flow that stems from a call/event handler. A pointer to that specific code and any supporting documentation, or a related plugin that I could work from (or possibly just use without having to reinvent the wheel) would be greatly appreciated!