what is the legend in those graphs, exactly?
The curves labeled alpha=3 through 7 (degrees) show cL/cL0, the ratio between the lift coefficient at a Mach to that at M=0 for the particular AoA or, if you like, the lift coefficient in real air divided by that in incompressible flow.
The red line is qc/q0, the ratio between impact pressure (pt - ps) and dynamic pressure
½ρV².
Perhaps you should ignore the thin blue (highest) line, it is the pressure correction pc/p0 according to Prandl-Glauert for small perturbations, e.g. thin airfoils at small angles of attack.
Then why don't we use an indication of that magnitude to fly an airplane?
Well, the ASI provides just that, it indicates CAS, which is aerodynamically more relevant than EAS, as illustrated in the graph.
Why do we use two instruments (altimeter and ASI) instead of one?
You're not interested in altitude?