Also shown on the map is a typical steady state working (or operating/running) line. This is a locus of the operating points of the engine, as it is throttled.
Being a high pressure ratio device, the working line is relatively shallow. If the unit had no variable geometry, there would be handling problems, because the surge line would be very steep and cross the working line at part-flow.
During a slam-acceleration from a mid-throttle setting, the compressor working line will move rapidly towards surge and then slowly approach the steady state operating point, further up the map. The reverse effect occurs during a slam-deceleration. These effects are caused by the sluggish response of the spool (i.e. inertia effects) to rapid changes in engine fuel flow. Compressor surge is a particular problem during slam-accelerations and can be overcome by suitable adjustments to the fueling schedule and/or use of blow-off (bleeding air off the compressor, for handling purposes).
In the particular example shown, a slam-acceleration from ground idle would cause a high-pressure compressor surge. Opening the blow-off would help, but some changes to the variable stator schedule might also be required.
Because a high-pressure compressor sees the choked flow capacity of the high-pressure turbine, the compressor working line is hardly affected by flight conditions. The slope of the working line approximates to a constant corrected outlet flow.