Feed one sine wave to the horizontal axis and another to the vertical, and the dot traces a Lissajous curve: an ellipse when frequencies match, a figure eight at 2:1, tightening knots as ratios grow complex. Jules Antoine Lissajous drew them with mirrors on tuning forks in 1857.
The curves made frequency relationships visible a century before FFTs, and engineers still read phase and ratio from them at a glance.
They are also a founding image of audio visualization: oscilloscope music, drawing pictures with sound alone, is composed Lissajous craft, and every X-Y mode display, including a vectorscope on a mix, is the same idea listening to real music.