Move away from recharts

• Recharts seems a bit clunky even though it works.
• This issue. It has been fixed already, but has yet to be published to npm.
• This will block an update to React 17.x, which I would like to be able to do eventually.

visx looks interesting, but it'll be a lot of work because I don't really know anything about d3. But it looks low-level enough to facilitate high customizability, which is something we do need.