Tinderbox (http://www.eastgate.com/Tinderbox/ http://www.eastgate.com/Tinderbox/) has a "map" view that functions somewhat like this, with two twists: 1. Notes

Tinderbox (http://www.eastgate.com/Tinderbox/ http://www.eastgate.com/Tinderbox/) has a "map" view that functions somewhat like this, with two twists: 1. Notes on the map can be connected to each other via arrows that show relationships 2. Notes can act as containers for other notes, so you're not limited to the 2D plane the author describes -- you can "zoom in" by opening a note and you'll be presented with a map view of that note's contents.