Somehow, I don't see the mention of the Vega visualization grammar [1] as a potential charting library. I think it is incredibly well-done, and is generally intuitive in its API. I've mostly used it through the Python bindings called Altair [2]. The good part is that that general grammar carries over to the browser as a full JSON spec that can be used directly in any language that supports JSON and has a Vega binding (which is pretty much all popular languages).

Having said that, I had to unfortunately abandon it because the ad-hoc control with Matplotlib [3] in Python is just infectious. Visual manipulations are far less easy to do in Vega. Being in JSON is also a restrictive though, because it is less interpretable by unstructured bots, where charts.css probably excels by design.

[1]: https://vega.github.io [2]: https://altair-viz.github.io [3]: https://matplotlib.org