What we all need to remember is that the same film and a different developer combination can generate a dramatic and vastly different density / exposure relationship from each other.
Originally Posted by David A. Goldfarb
T Max 400 in Xtol or Pyrocat at regular dilutions and processing is as you said, a straight 45 degree line.
Dilute Pyrocat with T Max 400 utilizing stand or semi stand process generates an extended long toe and an emphasized S shoulder to such a degree that you would swear that this could not be the same emulsion. I am sure that there are many other examples that fit this model.
As a result I am convinced that photographers that really know their materials can make quality photographs with virtually any film and developer combination. Looking at a graph is "interesting" but prints are the only thing that really matter.