I really like where you are going with this.
Maybe there could be a toggle switch in the Display section of graph settings to create a dotted connection between them.
Or, perhaps with either an automatic toggle or by creating a new tag called */History (or something legal), this new shared subtag node would be connected to all tags that had the History subtag.
I am having trouble finding it, but I had a similar feature or plugin idea using the same concept but for headings. In my idea, all notes that had a matching heading would be linked to a note with that heading name or a new note if one did not exist. Of course this would not be automatic and would have to be invoked.
Thanks.