Graphing: provide a matplotlib backend that plots to a browser

As an instrument scientist, I would like the option of a matplotlib backend that plots to a browser in a non-blocking way (rather than plotting into IBEX).

This is an easy extension of the backend that we've already done, which already serves a non-blocking web page.

An example use case for this (from LARMOR) is:
- I want my IBEX client pointing at the DAE perspective so that I can see spectra etc
- I am running my script in a standalone python window
- I want to be able to have non-blocking plots, entirely separately from the IBEX client.

Requested by Adam Washington