Add different plot backends

[jo-mueller]

I noticed that in some usecases (~10e6 datapoints) the plotting becomes quite slow, which can make the experience a bit tedious. Sooo I started woorking on making the biaplotter backend-agnostic.

Two things are needed for that:

• An abstraction layer for the axis object upon which is drawn
• An abstraction layer around the plot artists through which the plot propertes (point size, alpha, etc) can be controlled.

First look soon 👍

[zoccoler]

Indeed, matplotlib backend would be slow for these amounts of data, especially the Scatter artist.

I have some work started with pyqtgraph, which could be a good alternative option for testing other backends, just ping me when you have something ;)