A lot of things have changed since Sourcetrail was called Coati (about 2 years ago). We put a lot of work into improving indexing speed, handling multiple configurations/different languages within one project and reducing "sluggishness" in the UI. Sourcetrail runs now smoothly on code bases with multiple MLoC.
But I agree with your suggestion regarding data-flow analysis. That is what understanding unfamiliar source code often really comes down to. We also had some user requests to go that direction. We never really looked into this area so far, because it is a lot harder to collect the data (dynamic analysis) and it needs a whole new user interface.
While the data collection is solvable (Visual Studio debugger can do it, I think), I'm not sure whether it is really possible to come up with an effective user interface that shows which paths the different values take.
To explain why this is hard, let me use a metaphor: With data-flow you need to deal with a new dimension: time. Sourcetrail can handle dependencies between definitions really well, before the code is executed: space. What you want is a tool that combines the two into a space-time exploration tool of source code. Not sure if possible at all, but very interesting to think about. :)