There is no Early Combat. And my enemies had no navy to speak of, so my Barbary Corsair had no targets.
At T450 (or so), there is a spiking event, which I assume is when I decided to knock out a City.
Of more interest are the long stretches of 3-5 Combats in a Turn, when I selectively Attacked only with my Knights; and later, only with my Legio X (at three Attacks per Turn).
There is little need to be in any hurry. The AI never comes back after being pummelled. And in fact, after losing their first City (or two) barely manage to field any reinforcements. So, I can take my time, harvesting Experience with my Siege (something not shown on the graph) and funnelling ALL OF THE EXPERIENCE into a few select Units.
Sure, it hurts when those Massively Experienced Units die. But I've gone through Stacks at 100% Odds. So, such tragic occurrences can be minimized.