mcplot/mxplot: Expose canvas option/s to the interactive plotter.

[ebknudsen]

It could be useful to allow dynamically switching of invert canvas while interactively plotting.

[willend]

Hmm, not sure if we can easily do it "interactively", but we could easily have either

1. An option to centrally configure this in m[cx]gui
2. A shortcut-key that would exit the tool and rerun with that option set...

(the latter only relevant for the pyqtgraph tools, as e.g. the webgl plotter is browser/file based only)