I understand your point. I try to implement it as a web of things though as indicated by arrows going through and between each of the nodes. Each node is a tree yes, but together they form a connected web of nodes and trees where each node is another graph that can go as deep as it wants.

And I do agree that linear algebra is not just a subset of math but is connected to all the varying disciplines and I try to indicate these connections with arrows. Most of the important ones at least, otherwise the graph will just get messy.

The idea is to expose knowledge in a visual way and attempt to solve the problem of discoverability on the web.