Mindmapping your bubble diagram

https://bubbl.us/
For the Mac, there is MyMind, a mind-mapping (diagramming) app which converts to and from an outline. Linux has Freemind, VYM(ViewYourMind), Labyrinth, and Semantik, which don't seem to be as good as MyMind. Commercially, there's Inspiration for Windows. Somehow I missed the web-based option of bubbl.us until recently. You can export to png or jpg without creating an account, and control the colors and font sizes easily.