Shouldn't it be the other way around?
180 degrees should be the engagement envelope for a head on engagement, while 0 degrees should be the engagement envelope for a tail chase.
The engagement envelope for a head on engagement for a AAM should be the highest value, while the engagement envelope for a tail chase engagement should be the lowest value.
I think that perhaps the azimuth readings such as 180 degrees might refer to the flight direction of the target relative to the launcher. So 180 degrees is the target head on, where the missile range would be at maximum.
Well, perhaps other more knowledgeable folks might read it differently, but to my simpler mind it does seem like 180 degrees head-on, 0 degrees tail chase (so perhaps i was wrong about firing to the rear theory). So ranges look something like around ~270-280km head-on, ~120-125km or so tail-on for PL-16, ~180-190/100 for PL-15 and ~55-60/25-30 for PL-12.
But again, others might have a different/better interpretation.
PL-12 is the tiny circle in the middle? I wonder which version it is — earliest version has much smaller envelope than PL-12AE.