A large monolithic design does not scale well as new graphing or monitoring requirements are levied.
Observers register themselves with the Subject as they are created.
Whenever the Subject changes, it broadcasts to all registered Observers that it has changed, and each Observer queries the Subject for that subset of the Subject's state that it is responsible for monitoring. Some auctions demonstrate this pattern. Each bidder possesses a numbered paddle that is used to indicate a bid. The acceptance of the bid changes the bid price which is broadcast to all of the bidders in the form of a new bid. Chain of Responsibility passes a sender request along a chain of potential receivers.