How are compressor maps drawn?
From my textbook, Fluid Mechanics and Thermodynamics of Turbo-machinery by Dixon, it is explained that they find the results for the working line empirically. This corresponds to the line with the black dots on the diagram you provided.

Essentially, your test rig is made up of a compressor that is connected to a downstream throttle. It is the size of the throttle (in a real engine, this corresponds to the size of the nozzle) that is going to determine the test working line (called an operating line if it does not pass by the Design Operation Point). In other words, a different operating line corresponds to a different size throttle.

Then you vary the speed and measure the obtained pressure ratio. Connecting the dots will give you the working line.

ADDED DETAIL:
You have to know the size of the throttle because it will give you valuable information regarding your exit configuration. This has to do with the fact that the nozzle will usually be chocked, which will fix your non-dimensional mass flow through the nozzle. But that's a whole other topic in itself!

I did my best to answer your question, sorry if it does not help, I am only a student! If you want details of the calculations for compressor maps, you should check out chapter 5 of Dixon.