Replace c3.js

It seems that [`c3.js`](https://github.com/c3js/c3) [won't be maintained anymore](https://github.com/c3js/c3/issues/2831) and will be replaced by [`billboard.js`](https://github.com/naver/billboard.js) (see https://github.com/naver/billboard.js/wiki/Why-we-decided-to-start-billboard.js%3F). We should decide on whether to migrate to `billboard` or another solution.