Remove plotly backend

[nvaytet]

The plotly backend, activated via pp.backends['2d'] = 'plotly' is basically unused.
It only has limited support for 1d line plots.

In addition, it is slowing down addition of new features (e.g. #514 ) and adding to maintenance burden.
We remove it for these reasons.

Moving forwards, we should be careful not to have matplotlib specifics creep in to part of the codebase which should be plotting-library-agnostic.

[jl-wynen]

Moving forwards, we should be careful not to have matplotlib specifics creep in to part of the codebase which should be plotting-library-agnostic.

How relevant is this really? Can we commit to matplotlib fully and simplify the code?

[nvaytet]

How relevant is this really? Can we commit to matplotlib fully and simplify the code?

That is something we should maybe think about. I guess it could simplify quite a bunch? We should still keep some clean separation between 2d and 3d plots (where we still want to use pythreejs), but otherwise I'm guessing matplotlib will live as long as python lives?

[nvaytet]

Conclusion from in-person discussion with the DRAM analysis team:

The plotly backend in plopp is not needed by any current package. The apps that use plotly do their plotting manually with the library directly, and the ones that use plopp make use of matplotlib.

There are some useful parts to Plopp other than just the displaying of figure (data preprocessing, slicing, and a graph of connected nodes to interact with widgets). This part may still be useful even if you are not interested in matplotlib.

We decided that we could create a mode where instead of creating a figure, we output a json blob which contains all the information required to make a plot. Then, the groups are free to use whatever library they want to plot from the info in the json. It would be like serializing the plots.

For the record, this idea was inspired by #203 .