Excel is not well designed for contour mapping, and you may want to try
another package instead. If you can calculate the efficiency levels for
a range of pressures and flows, you could use the "3-D Surface Chart"
type, but the results will be blocky, and integrating it with your
original line curves will be difficult.
If you're not prepared to abandon Excel or 2-D XY charts, you could use
one of the third-party chart labeling utilities. Google "excel chart
labeler" and "excel chart tools" to find the two popular free add-ins by
Rob Bovey (AppsPro) and John Walkenbach ("J-Walk"). This will let you
show the efficiency of each point on the pressure-flow curve as a
number.
Alternatively, are you prepared to abandon the compressor map
arrangement of pressure against flow, and plot efficiency against flow
instead? You could use the technique above to label the curves with the
pressure at each point. It might be worth showing two graphs next to
each other, one pressure-flow, and the other efficiency-flow.