Select which values to plot

[dandavies99]

This is more of an advanced feature (definitely a "nice to have").

Users should be able to select which values to plot. This would, in its most basic form, probably take the form of two dropdowns, one for y1 and one for y2, to select up to two variables to plot against time.

The main challenge will be reimplementing how plots are rendered. Currently the plots are rendered as HTML divs using so are not truly interactive in this way.

Aha! Link: https://imperial-college-ict.aha.io/requirements/RSE-138-2