Fourier analysis
The Z axis is a fourier compound: a whole-octave step in X read as a sum of harmonic parts.
classic source
Théorie analytique de la chaleur
Joseph Fourier · 1822
Original statement of decomposing any periodic quantity into sines and cosines.
modern reading
Who Is Fourier? A Mathematical Adventure
Transnational College of Lex · 1995
The friendliest illustrated route from waves to the Fourier transform.
The Mathematics of the Heavens and the Earth / Fourier Analysis: An Introduction
Stein & Shakarchi · 2003
Clean modern textbook treatment when the popular account stops being enough.
The Scientist and Engineer's Guide to DSP
Steven W. Smith · 1997
Free online; spectra, windows and the DFT in practical terms.