Why there is a 'MACH' in this Compressibility Correction Chart?
As I read it, the upper arch pointed to by Mach 1.0 represents the correction necessary at the different pressure altitudes when flying at Mach 1.0, while the lower arch pointed to by Mach 0.8 represent the correction necessary at those pressure altitudes when flying at Mach 0.8.

i.e. the correction necessary at 30,000 feet at Mach 1.0 is roughly -30, while the correction at that same altitude but Mach 0.8 is only roughly -15.