Can i assume this is one round per charge weight increment? If so, that flat spot at 26.4 - 26.7 could be real or an artifact. After all you’re dealing with a sample of one (never a good thing). I prefer OCWs, ladder type tests aren’t reliable as a whole. Id do a ladder, see a flst spot then do the same ladder a day later and see a completely different flat spot or none at all.
At what charge weight did you see overpressure signs, if any of them?