I am wondering though, how the degree of roasting affects this diagram. I am using a dark roast on the Strega and am having problems with bitterness. According to my understanding of this diagram I was grinding finer, thinking higher solubles yield would give me more sweetness to balance the bitter/sour. This didn't work out and lately I was given the advice to grind coarser, which has improved things (I am still working on the temperature and will maybe get a PID).
Reading Jim Schulmans "Some aspects of espresso extraction" written in 2007 got me thinking. Jim talked about two bitter components, the light maillards and the dry distillates. Maybe my problem are the dry distillates dominant in dark roasts. These very slowly dissolving flavors can seemingly be reduced in the shot by lowering the solubles yield. For a given setup (espresso-machine, grinder, basket ...) this means coarsening the grind and adjusting the dose accordingly. My cup would then still contain the fruit acids, maillard compounds and caramels.
So maybe in dark roast coarsening the grind and reducing the solubles yield gets you sweeter shots, because the bitter (dry distillates) is reduced and in lighter roasts tightening the grind and increasing the solubles yield gets you the sweeter shot by increasing the caramels. Bear in mind, that I am a newbie theorizing here, so if I'm way off track or am misunderstanding things please help me.