A sub-plot shows the variation of isentropic (i.e. adiabatic) efficiency with flow, at constant speed. Some maps use polytropic efficiency. Alternatively, for illustrative purposes, efficiency contours are sometimes cross-plotted onto the main map.
Note that the locus of peak efficiency exhibits a slight kink in its upward trend. This is due to the choking-up of the compressor as speed increases, with the variable stators closed-off. The trend line resumes once the variables start to move open.