I haven't used roam, but the screenshots made me wonder if there might be some overlap with TiddlyWiki, "a unique non-linear notebook for capturing, organising and sharing complex information."

TiddlyWiki can be as simple as a single HTML file, but can also work with a server.

There are also quite a few extensions, including TiddlyMap[1], which adds concept mapping.

[0] https://tiddlywiki.com/

[1] http://tiddlymap.org/