Complex/Hybrid data structures- do people ever combine structures like graphs and hash tables together?

I just built a LRU cache that combines a hash table and a double linked list. To me it was a brilliant idea to combine those two structures and use the strengths of each together, so it got me thinking, can people or do people combine hash tables with other more complex data structures like trees and graphs? Are there use cases and what would they be called? I don’t see much written about these on the usual sites like Wikipedia.

Obviously the hash table gives constant lookup time (or nearly so allowing for collision resolution), and the tree or graph gives you ways and means of doing traversals.

I know you can hang a new tree’s parent node from every single hash entry but I mean having one tree main tree or graph overlayed and interconnected with one main hash table