While typical syslog configurations are rather simple and easy to understand, complex ones can be challenging. The graphing capability we now have provide a high-level, human-readable representation of rsyslogd's internal control structures. The beauty with that is that every user can create an exact right diagram from his own configuration.
I hope this is a useful tool for documenting a system setup, but I also think it is a very valuable tool for learning to understand rsyslog as well troubleshooting problems with message processing.
With that said, I now send you to the new graphing feature manual page, which I hope provides sufficient insight into how this feature is used.
But... here is a sample graph to whet your appetite: