Actually, that's a feature that was common going all the way back to the very earliest image file formats: https://en.wikipedia.org/wiki/Indexed_color

It'd be easy enough to make the chart a plain old GIF or indexed PNG; the only non-trivial part is that you'd need add some code to the page it's embedded in to swap out the color palette. (You could also make it an SVG and manipulate it even more easily using the DOM.)