Replace ChartJS with native CSS based charts

[cabrerahector]

ChartJS is a great library for rendering charts on your site. However, it's yet another dependency I need to keep my eye on and update when/where necessary.

Nowadays it's possible to build CSS based charts via the attr() function.

Let's see how this works and how it fares on different browsers as I believe that, at the time of writing, attr() is still not widely supported.

As an aside, we could also update WPP's themes that currently rely on style attributes (iirc) to render in a certain way.