There's no plugin that I am aware of, but there are a number of free websites that can be linked from a course page (or even embedded as a url link)
2 of the plugins on the search linked to were activity modules, 1 is a course format and 1 is a block. So those last 2 would be accessed in a different way to an activity/resource.
Did you try the Advanced Mindmap? I haven't tried any of them but from that plugin's write up:
"This Advance Mindmap module allows you to create mindmapping activity in moodle. Added features include: - Every user now have a separate mindmap in one single activity..."
No idea if you can grade them though.
Good to hear you're getting closer to a solution. Re grading, again, something I haven't used but you could always set up a manual grade item.
Not the best workflow but as far as I know the only workaround at this time for what you are looking to do.