Previously reported problem with Tidgraph not rendering correctly in open windows and in modals does not seem to be an issue with Viz (for GraphViz) plugin.
Since I just want a diagram (without tiddler links) and this works with windows, I’ll be using this to report on anything I want to visualize.
Tiddler A includes Tiddler B. Tiddler B includes Tiddler C.
Tiddler A includes Tiddler C.
Diagram works. Cool.