In the diagnostic a refrigeration or AC system, the P-H diagram will have little value in comparison to a P-T table for refrigerant R-22. Sure you can plot the cycle on the P-H diagram but you'll need the temperature data to go along. Once you have the cycle plotted, then you can estimate heat release thru condenser, heat absorbed by evaporator, heat of compression from compressor from which you can determine horsepower as long as mass flow rate is know, and the performance of the thermostatic expansion valve or capillary tube.