Essentially the same lines as your windmills show between Q1&Q4, I'm moving those two quadrants apart and superimposing possible negative curves between them. There is also one other big difference, the lines are meant to show the photo I want at either end regardless of negative exposure.
Unlike the Dorst diagram I'm not scaling to the density of the medium so there is no funnel effect and unlike windmills no corners to turn, not trying to give that much info. The range shown/defined at each stage: "scene", "film", and "paper"; is simply "1", "a print's worth" or "100%" of the print range.
Regarding the "under" curve I've drawn "it does not get enough camera exposure" to create detail on the negative for my chosen zone III subject matter, it is showing how a failure/operator error/scrimping on exposure might lose detail. The film curve in my diagram remains pegged to where my chosen subject matter falls or in this case fails to fall..