Sourcetrail is now free and open-source software
"Sourcetrail is a cross-platform source explorer that helps you get productive on unfamiliar source code. It uses static analysis on C, C++, Java and Python source code and lets you navigate the collected information within a user interface that interactively combines graph visualization and code display."
It is said to be able to cope with the huge #LibreOffice code base, so it can't be too bad ;-)
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