another_jim wrote:In this diagram, if the fast grind works for espresso, the slow down wundergrind is probably a gusher. The wundergrind has a coarser, more tightly distributed large particle distribution and less fines. Therefore, it probably resists the flow less and will gush. Note that tighter distributions and less fines is also what happens when you adjust the grinder more coarsely and do not change the speed.
As Jim notes, finer grinds cause more bean breakage and generate more fines, leading to increasingly bimodal distributions. But it's possible that changing rpm is another parameter to be manipulated. For example, instead of coarsening the grind setting for non-espresso brewing, you might decrease the grinding speed. Either of these changes would coarsen the grind size and generate a more unimodal distribution.
Perhaps these factors (grind setting, bean load, grinder rpm) are really just one parameter, and by careful manipulation of grind setting you can get the same results as changing rpm. There's no way to tell from the plot.