Static sizing of Matplotlib charts

[micahtyong]

Currently, we generate Matplotlib charts as SVG images on the lux-org/lux backend then pass them into the lux-org/lux-widget frontend. From there, we render them as raw images. Because of this constraint, we aren't dynamically resizing the Matplotlib charts as we do for Altair (#333). Our current fix is to obtain the plotting_scale then compute the width and height of the image (rather than letting the parent div + flexbox do the work for us).

Here are some guiding questions to get started on this issue. In either case, we would need changes in both the frontend and backend.

1. Is there a component similar to <Vega ... /> in Matplotlib? If so, we can use that component in the frontend and use that instead of generating an SVG.
2. Is there a way to only tweak the frontend using Flexbox styling? If so, we can use plotting_scale from the backend similar to how scaling is accomplished for Altair charts.