one use of an x/y mode of scope (vectorscope)
Audio 
In audio applications, a vectorscope is used to measure the
difference between channels of stereo audio signals. One stereo
channel drives the horizontal deflection of the display, and the other
drives the vertical deflection. A monaural signal, consisting of
identical left and right signals, results in a straight line with a
gradient of +1. Any stereo separation is visible as a deviation from
this line, creating a Lissajous figure. If a straight line appears
with a gradient of −1, this indicates that the left and right channels
are 180° out of phase.