These graphs could show the movement of a string, and electrical signal, the movement of a speaker coil or the movement of the air.
These frequencies are related, but they don't need to be. Just plot the signals and add them together and the resulting graph plots out what you get.
Sometimes what you get has some big, fast changes or really slow ones. These may bet 'filtered out' by the characteristics of the circuit or loudspeaker etc. and this can 'colour' the sound.