Motivation

Implementing a new plotting backend is a tedious task, since it demands to rewrite plotting functions for all objects with all their style attributes. Also the collection logic has to be implemented every time.

New Approach

In this PR, a new approach is considered. In this case a 'generic' backend is defined, heavily based on the plotly trace syntax. The generic backend function returns a list of traces (as dict) with declarative properties, hence no need for knowledge of the internal magpylib logic of style, path, collections. The only thing to implement is a translator for each new backend, which is in most cases fairly straight forward and requires very little additional code.
One more advantage, is that for every new object of the the magpylib library, only one graphic implementation needs to be implemented and backends don't need to be updated.

Implementation

For plotting only two types of traces are really necessary: 'scatter3d' and 'mesh3d'
For a simple example the generic traces looks as follows:

{'type': 'mesh3d',
 'i': array([0, 0, 1, 2]),
 'j': array([1, 1, 2, 0]),
 'k': array([2, 3, 3, 3]),
 'x': array([ 1.,  0., -1.,  1.]),
 'y': array([ 0.,  1.,  0., -1.]),
 'z': array([-0.70710678,  0.70710678, -0.70710678,  0.70710678]),
 'colorscale': ((0.0, '#00b050'),
  (0.16, '#00b050'),
  (0.26, '#dddddd'),
  (0.74, '#dddddd'),
  (0.8400000000000001, '#e71111'),
  (1.0, '#e71111')),
 'intensity': array([0.5, 1. , 0.5, 0. ]),
 'showscale': False,
 'name': 'Tetrahedron',
 'color': '#2E91E5',
 'opacity': 1,
 'legendgroup': 'Tetrahedron(id=140663769004688)',
 'showlegend': True}

{'type': 'scatter3d',
 'mode': 'lines',
 'line_color': '#2E91E5',
 'opacity': 1,
 'x': array([0.25, 0.25, 0.13, 0.25, 0.37, 0.25, 0.25,  nan]),
 'y': array([0.        , 1.99999998, 1.79999998, 1.99999998, 1.79999998,
        1.99999998, 1.99999998,        nan]),
 'z': array([ 0.,  0.,  0.,  0.,  0.,  0.,  0., nan]),
 'legendgroup': 'Tetrahedron(id=140663769004688)',
 'showlegend': False}

Tasks

• implement generic backend
• matplotlib translator
• plotly translator
• solve custom model3d handling (custom extra traces attached to any magpylib object now become obsolete)
• deal with magic syntax in generic trace to avoid special cases in every translator (e.g. line_width=2 vs line=dict(width=2))
• update changelog
• update docs
• update tests

Notes

• This PR also implements matplotlib animation (Add animation for matplotlib backend #428)
• This new feature changes the look of the matplotlib backend, which is now more in line with the plotly backend