I don't see what you mean. The left axis is a measurement of cost per gram of protein. The bottom gives you a measurement of density. So anything lower on the chart is cheap for what you get and then the further right you go the smaller the portion required to consume to get that amount of protein.

How do liquids differ from dry foods in terms of protein stuff? (Waves hands vaguely).

I was mildly surprised that milk is way down in the bottom left quadrant.