Freeplane, a fork of the mind-mapping program, Freemind, works really well. Except for the fact that in my (K)Ubuntu installation, it sometimes hangs in the middle of editing a node. Once it becomes non-responsive, the only fix is to kill the process and restart the program. The version of Freeplane in Ubuntu 18.04 is 1.6.13. However, the current version (March 2019) of Freeplane is 1.7.7. So, it's highly possible that upgrading Freeplane to the latest version will fix this issue.