A vectorscope draws the two channels of a stereo signal against each other, one on each axis, rotated 45 degrees so mono material stands as a vertical line. Width becomes visible: correlated content leans upright, wide content blooms sideways, and phase problems sprawl horizontal.
Engineers glance at it to answer one question fast: will this survive mono? The shape says so before the ears confirm.
The display is also simply beautiful, a live drawing made by the music’s own geometry, which is why it appears in Photism’s signal scene. Same math as the Lissajous curve, pointed at a mix instead of test tones.