I am confused. Whatever the graph represents, it seems to be the best at ISO 64, from which it gradually declines (getting worse) and then suddenly drops at 3200 (i.e., getting much worse). So, my layman conclusion is not to set the camera to 3200 or more, but to stick to up to 2500. That appears to be the opposite of what you are saying. I am sure I must be wrong, but where?