I am with Mr Coyle in being confused by this format although I've used it (successfully, I guess) for 30 years. What happens when the line from observed parameters (bottom up) aligns with the line from critical parameters (top down)? I have always rechecked to break the tie.
It is really hard to be precise plotting this as one must parallel curves and the marks made have width. Digital, a spreadsheet for instance, would be concise.
My best guess is that Gallic logic required that plotting observed line and decision line points for intersection be 2 different operations.