XAnalyser is a program to analyse a stereo audio signal. It has two 
displays:
Frequency Spectrum

Using Fast Fourier Transform, the time domain of the signal is 
transformed into the frequency domain, i.e. the amplitude (in 
logarithmic scale) of the audio signal is plotted versus the 
frequency. Either the sum of the left and right channel of the 
audio signal can be shown or both channels simultaneously.

XY Scope

Roughly speaking, the audio signal of left channel deflects a 
point horizontally and the right channel vertically (just as the 
beam of a CRT would do). Thus, an audio signal only present on 
the left channel produces a horizontal line, whereas an audio 
signal only present on the right channel produces a vertical
line. A mono signal produces a 45 degree line. A stereo signal 
creates a wilde pattern (if the phase is correct, predominately 
in the same direction as a mono signal) or may even fill the entire 
scope.