From memory. Pressure ratio on the y axis, air flow on the x axis. You then have the lines representing turbo rpm and the 'island' regions relating to efficiency. If you have an idea of the bhp required and boost you want to run you can start looking at Maps. You can then see for the pressure and air flow where the turbo is operating. If it operating far from the central island you will need to change the turbo spec. There will also be a surge line.

This could mean a completely different turbo or modifications to trim, exh housing ect.

This is how I understand it, and welcome any corrections.