It is what the windmills show but not how they show it, the windmills IMO are many times tough to follow and compare. They also tend to lead to a single best exposure wins conclusion rather than showing where latitude exists.
The tone reproduction diagram shows the original subject, camera image, negative characteristic curve, paper characteristic curve, and the reproduction curve. There's no place for latitude to hide.
From your original post, it sounds like you are basically talking about the tone reproduction curve.