Thanks John! At least now I know the relative meanings of the shading and the graph. I have gotten the "excellent" reading a couple of times and I obviously knew what to strive for but it is good to know specifically what I did to achieve it!
I don't intend to become obsessed with MPG (read-hypermiler I ain't) but it is good to know that very good mileage is a given and excellent mileage can be a goal.