Use matplotlib instead of plotly in Explorer

Plotly has the advantage of resulting in interactive plots in a jupyter notebook, but it is does not result in easily portable plots. We should consider ways of making the exploration portable, so that a set of plots could be, e.g., batch-generated from a summary.

The obvious choice is to fall back on `matplotlib`, which is already used in the first place to generate many of the plots that then get converted into plotly plots.