What software was used to create the diagrams in the Git Community Book? [closed]

The diagrams in the Git Community Book are excellent and I'd like to know what software was used to create them. I emailed the author, but he didn't respond, so does anyone else know?

(This question was raised by request within this stackoverflow answer).

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yEd is a powerful and free graph editor that produces really nice graphs.

I use it often for my needs. It provides automatic layout and many shape / line options.

These graphs could have been produced by this program.