Unified Graph Views with Multigraph by Joshua Shinavier.
From the post:
Next in line in this parade of new Blueprints utilities: a Graph implementation called MultiGraph which has just been pushed to Tinkubator:
https://github.com/tinkerpop/tinkubator
MultiGraph wraps multiple, lower-level Graph implementations and provides a combined graph view, unifying vertices and edges by id. So, for example, if you have a vertex with an id of “Arthur” in graph #1 and another vertex with an id of “Arthur” in graph #2, and you put those graphs into a MultiGraph, the unified vertex with the id “Arthur” will have all of the properties of either vertex, as well as all of the edges to or from either vertex. Any vertices and edges which exist in some graphs but not in others will also exist in the MultiGraph view.
Using ids to trigger merging and precedence to cope with conflicting values for edges. I don’t think “precedence” is going to be very robust in the long run but every project has to start somewhere. Preserving provenance after merging is likely to be a requirement in many applications.
This and similar discussions happen at the Gremlin-Users group.