At one time there were at least 3 programs available for doing Mind Mapping on Linux: FreeMind, XMind and FreePlane. FreeMind is the grandaddy of mind mapping software. It's written in Java but is open source and still being developed. FreePlan is a fork of FreeMind that adds some nice features and is at any given time a bit ahead of FreeMind in its development cycle. XMind is also a Java desktop application that started out as an open source project, but whose latest version (XMind Zen) is now closed source with access via a monthly ($4.58) subscription.
In trying out all three recently I discovered the FreeMind is no longer available in the Ubuntu 18.04 repository, and that the version (1.6.13-1) of FreePlan in the repo freezes on use.
The good news is that I have a gist on installing FreePlane (my current favorite) on Ubuntu 18.04 LTS here. Same for here for FreeMind. Finally, I've devoted a whole repo to making XMind 8 work here.