Visual testing

This is a reminder to myself that we should set up some image-based testing of our outputs as part of the testing suite. Testing that the Plotly traces have the right properties goes a long way, but we should have some way to check that the final product looks correct.

Plotly supports [static image export using Kaleido](https://plotly.com/python/static-image-export/#interactive-vs-static-export). We could use this to generate static output (e.g. a PNG, rather than interactive HTML) and compare to reference images. The HTML pages also support the ability to save a PNG - we should check whether this just calls the Kaleido static export, or is something different (and if the latter, see if we can activate that in a test).