Oscilloscope

Try setting the frequencies to various fractions of each other. Then, add small variations.

An oscilloscope is a type of electronic test instrument that allows observation of varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time [wikipedia].

This web experiment displays the combination of three signals. Specifically, three sine waves, each with only an x, y, and z component respectively. The resulting patterns you see are called Lissajous curves.

There are technical challenges that are hard to surmount when building something that displays frequency-based images. For example, the framerate of this browser acts as an unwanted fourth signal that affects the curve.