I absolutely love watching visualizations. I've been following various YouTube channels on the topic such as 3Blue1Brown and SmarterEveryDay and thought it was so cool to see waves represented as a series of end-to-end pendulums navigating around a circle. I thought it was even cooler when I found out you can draw any line shape using this method. Thanks to The Coding Train for getting me back into this topic enough to play with my own version of it.
Above is a rough approximation of a square wave, which you can adjust using the slider to a plan sine wave by moving left, or a better square wave by moving right. You can also click and drag in the right partition of the window to draw your own shape! When you release the mouse, it'll start re-drawing it using the Discrete Fourier Transform. Pressing Escape gets you back to the sine wave demo. Neat!