Make plots interactive elements on the website using Altair

If [Altair](https://altair-viz.github.io/) is used as a backend we can make the visualizations on the website be naively interactive, even though the webpage will be fully static. :+1: 

Examples:

* [Interactive tooltips](https://altair-viz.github.io/gallery/multiline_tooltip.html)
* [Interactive legend](https://altair-viz.github.io/gallery/interactive_legend.html)