feat: Add 'olli' renderer to generate accessible text structures for screen reader users

[binste]

Olli is an open-source library for converting data visualizations into accessible text structures for screen reader users. Starting with an existing visualization specification created with a supported toolkit, Olli produces a keyboard-navigable tree view with descriptions at varying levels of detail. Users can explore these structures both to get an initial overview, and to dive into the data in more detail.

Closes #3575. A possible use case of this is that in a university course where Altair is used in a Jupyter notebook, with a single line of code, alt.renderer.enable("olli"), all charts in a notebook can be made accessible for screen reader users.

I've tested this in JupyterLab and VS Code. In JupyterLab, all Olli controls worked, in VS Code, the following do not:

• T: Open table
• O: Jump to Olli tree
• F: Data filters

This might be an issue with how VS Code intercepts those key strokes. Might be configurable in VS Code but for now, I'm happy if it works in JupyterLab.

[binste]

@jonathanzong Would you mind reviewing the JavaScript code? It's in an ugly Jinja template without syntax highlighting but maybe you notice something when skimming through it. It works in JupyterLab and VS Code so it can't be super wrong.

I tested it with Mac VoiceOver and that worked fine but I have no experience with this. Example code you can use:

import altair as alt
import pandas as pd

alt.renderers.enable("olli")

source = pd.DataFrame({
    'a': ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'],
    'b': [28, 55, 43, 91, 81, 53, 19, 87, 52]
})

alt.Chart(source).mark_bar().encode(
    x='a',
    y='b'
)

[jonathanzong]

looks good to me!

[dangotbanned]

Seems like a really interesting feature

Just had one question, since your js got the okay in #3580 (comment)

[dangotbanned]

https://github.com/vega/altair/pull/3580/files#r1750840828

Sounds good to me, thanks for this @binste

[binste]

Thank you both for the reviews!