The green tree is the directory structure. Blue lines are function dependencies. Red lines are variable dependencies. Yellow flashes show file size modifications. Green flashes show files being moved across directories.
Red flashes show new files.
How this is done For each kernel source tree, C files are preprocessed and parsed. Functions and static variables are cross-referenced. This takes up to 15 minutes for a recent kernel. Layout is done with trivial spring-based techniques. Links are weighted according to how often functions are used.