Remove nvd3 dependency

It seems that our current [nvd3](https://github.com/novus/nvd3/) dependency (which is compatible with [d3.js](http://d3js.org/) version 3.5.3 and later) hasn't made a new release since 2017. Given that d3.js is now on [v7.8.5](https://github.com/d3/d3/releases/tag/v7.8.5), we should probably get rid of nvd3.