I have used Freemind for many years. Works with Mac and Linux too.
I tryed different mindmap softwares but they have all somekind limits unless pay 100 bucks or so. I tryed find some free (non browser one) and I found one which I like:
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Info: https://en.wikipedia.org/wiki/FreeMind
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Yes, Swing is a general purpose GUI toolkit. You can do anything you want. You will want a JPanel, override paintComponent().
As an example you can look at FreeMind which is mind mapping software written in with Swing, I don't believe it is maintained anymore but you can still download the source:
https://sourceforge.net/projects/freemind/files/freemind/1.0.0/freemind-src-1.0.0.tar.gz/download
http://freemind.sourceforge.net/wiki/index.php/File:FreeMind-computer-knowledge-080.png
I don't think there's one specifically made for gaming, or specifically anything, they're just mindmaps... Most people and especially companies will use either whiteboard or a piece of pen and paper (or at some cases software like https://sourceforge.net/projects/freemind/) and it only makes sense because no matter what you are trying to make the mind map is always the same