Have another upvote from me. This would seem to be rather the whole point of having nested tags. If you can’t use nested tags in the graph view, they lose the vast majority of their functionality. The whole point of tags is to group like things.
For example, I have the following tags: #Person/Work/Boss, #Person/Work/Co-worker, #Person/Relative, & #Person/Neighbor.
If all these people would clump near #Person, that would instantly organize my graph view immensely. As the groups (neighbors) and (bosses & co-workers) are generally unconnected to one another, having them automatically split out would again help me to see how the various groups of people are intertangled.
This is obviously a ridiculously simple example, but I believe it conveys the utility that this functionality would impart.