Problem reading and rendering Spectrum
The x-axis is indeed frequency, but the numbers are quite strange to me. One would expect a scale in powers of 2 (octaves) or of powers of 10 (decades).

The y-axis represents the relative magnitude of the signal at that particular frequency. The 0dB level is what you would get from a sine wave with the maximum amplitude (-1..+1). So your P2 is 1.0.