Improve support for Vega and VegaLite as format in save and mimetype

[mattijn]

Potential feature requests.

• 1. As was mentioned by @binste here https://github.com/altair-viz/altair/pull/2829/files#r1070558033

We could associate the file endings .vg and .vg.json with vega and .vl, .vl.json with vega-lite so the files can be opened in JupyterLab https://jupyterlab.readthedocs.io/en/2.2.x/user/file_formats.html#vega-vega-lite

• 2. As was mentioned by @mattijn here https://github.com/altair-viz/altair/pull/2829/files#r1086737782

Hm, I just noticed the following variations in the code:
https://github.com/altair-viz/altair/blob/1ed3f6f2fd3647bcf1f2e5e81dd328a0f9cf623d/altair/utils/mimebundle.py#L72-L77

It seems there is different type of json, namely, application/json and application/vnd.vegalite.v5+json (for altair 5). So I'll include vega-lite also as format, it probably would be better if it was renamed to something as vl-json.

• 3. As was mentioned by @binste here Remove vega v5 wrappers #2829 (comment)

Does it make sense to factor out this list into a variable and use it in the strings everywhere? Feels like this is repeated quite often. (red. FORMATS = ["html", "json", "png", "svg", "pdf", "vega", "vega-lite"])

This list should also be updated in the docstring of v5/api.py:TopLevelMixin.save

• 4. As was mentioned by @binste here Remove vega v5 wrappers #2829 (comment)

I can't comment on the correct line but vega does not work here. It will try to access the mimebundle with key image/svg+xml on line 169.

Currently there is no support for vega or vega-lite as output format. This could be supported, both options would be json, but vega would be a compiled version of the Vega-Lite specification with application/vnd.vega.v5+json mime type where the vega-lite would be application/vnd.vegalite.v5+json. This is linked to bullet number 1.

[binste]

For the save method, there are no mimebundles to be saved or returned so I think format='vega-lite' will be equivalent to format='json' except that we could extend the part where format is determined based on the file extension https://github.com/altair-viz/altair/blob/master/altair/utils/save.py#L84 so that .vl and .vl.json are linked to vega-lite, rest same as json.

[dangotbanned]

Adding a link to the stable docs: https://jupyterlab.readthedocs.io/en/stable/user/file_formats.html#vega-vega-lite

Docs

Vega/Vega-Lite

JupyterLab Vega-lite rendering (YouTube)
Vega:

• File extensions: .vg, .vg.json
• MIME type: application/vnd.vega.v5+json

Vega-Lite:

• File extensions: .vl, .vl.json
• MIME type: application/vnd.vegalite.v3+json

Vega and Vega-Lite are declarative visualization grammars that enable visualizations to be encoded as JSON data. For more information, see the documentation of Vega or Vega-Lite. JupyterLab supports rendering Vega 5.x and Vega-Lite 3.x data in files and cell output

@jonmmease would vl_convert.vegalite_to_vega be helpful here?

I just tried this out on https://jupyterlite.github.io/demo/lab/index.html for Vega-Lite:

import altair as alt
from vega_datasets import data

cars = data.cars()

alt.Chart(cars).mark_point().encode(
    x="Horsepower",
    y="Miles_per_Gallon",
    color="Origin",
).save("example1.vl.json", format="json")

With .vl.json suffix

With regular .json suffix

Click to open

Open with

[dangotbanned]

Marking this as breaking, since existing code that relies on the .json being added would have a different suffix if this was implemented