And that's why it doesn't really correlate. In the late 400s, East Rome and the Sassanids were doing just fine, not to mention the "Barbarian" kingdoms. And when you integrate periods, the gap around 476 smooths out. No-one even knew it when Rome fell, it was just a period of increased turmoil and slow institutional decline which was repeatedly slowed down and reversed. At any rate, we're talking about calling the Middle Ages the Dark Ages. The Medieval period wasn't even properly started when we see the lowest portions of the graph.