This graph type is typically used in hydraulic studies, particularly for fire suppression systems. The horizontal coordinates of the graph are scaled to the 1.85 power because, in the Hazen-Williams formula, pressure is proportional to the flow to the 1.85 power. The Hazen-Williams formula will therefore plot as a straight line using this scale type.

From Wikipedia:

The Hazen-Williams formula can be used to calculate the pressure loss in a length of pipe due to friction dependent on the flow. This equation is commonly used for pressure drop calculations in American fire sprinkler systems, water distribution systems, and irrigation systems.

where:

P_d=pressure drop in pounds per square inch / foot

Q = flow, gallons per minute

C = factor (friction loss coefficient) - the higher the C factor, the smoother the pipe. Typical C factors used in design, which take into account some increase in roughness as pipe ages are as follows:

Asbestos-cement - 140

Cast iron - 100

Concrete - 100

Copper - 130

Galvanized steel - 100

Polyethylene - 140

PVC - 150

d = inside diameter in inches