altair v6.0.0 breaking change

[Jhsmit]

It looks like altair v6.0.0 has changes which render use in solara broken

For example the docs example no longer works: https://solara.dev/documentation/components/viz/altair#figurealtair

Traceback (most recent call last):
  File "/root/solara/.venv/lib/python3.12/site-packages/reacton/core.py", line 1702, in _render
    root_element = el.component.f(*el.args, **el.kwargs)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/solara/solara/components/figure_altair.py", line 31, in FigureAltair
    raise KeyError(f"{key4} and {key5} not in mimebundle:\n\n{bundle}")
KeyError: "application/vnd.vegalite.v4+json and application/vnd.vegalite.v5+json not in mimebundle:\n\n{'application/vnd.vegalite.v6.json': {'config': {'view': {'continuousWidth': 300, 'continuousHeight': 300}}, 'data': {'name': 'data-7684334a68ab74ba4c3ca7d0ceaeb461'}, 'mark': {'type': 'bar'}, 'encoding': {'x': {'field': 'a', 'type': 'nominal'}, 'y': {'field': 'b', 'type': 'quantitative'}}, '$schema': 'https://vega.github.io/schema/vega-lite/v6.1.0.json', 'datasets': {'data-7684334a68ab74ba4c3ca7d0ceaeb461': [{'a': 'A', 'b': 28}, {'a': 'B', 'b': 55}, {'a': 'C', 'b': 43}, {'a': 'D', 'b': 91}, {'a': 'E', 'b': 81}, {'a': 'F', 'b': 53}, {'a': 'G', 'b': 19}, {'a': 'H', 'b': 87}, {'a': 'I', 'b': 52}]}}, 'text/plain': '<VegaLite 6 object>\\n\\nIf you see this message, it means the renderer has not been properly enabled\\nfor the frontend that you are using. For more information, see\\nhttps://altair-viz.github.io/user_guide/display_frontends.html#troubleshooting\\n'}"

Downgrading to altair 5.5.0 fixes the issue

[EwoutH]

Hi @maartenbreddels, I hope you had a nice Christmas break!

I was curious if you or your team could pick up this issue. Support for Altair 6.0.0 would be great, and if not feasible, pinning the version to a compatible release.

I still think a pre-release CI workflow could really help with catching these issues early:

• Add pre-release dependency testing to catch breaking changes early #1111

[EwoutH]

It looks like a Solara–Altair compatibility break caused by Altair 6 switching to Vega-Lite v6.

FigureAltair currently assumes the MIME bundle contains either:

• application/vnd.vegalite.v4+json
• application/vnd.vegalite.v5+json

With Altair 6.0.0, the mimetype emitted by the "mimetype" renderer is instead:

• application/vnd.vegalite.v6.json

As a result, Solara raises a KeyError even though a valid Vega-Lite spec is present in the bundle. Downgrading to Altair 5.5.0 works because it still emits the v4/v5 mimetypes.

A minimal fix on the Solara side would be to also accept the v6 Vega-Lite mimetype, or more generally select any bundle key that starts with application/vnd.vegalite. (possibly preferring the highest version).

After this change, Solara’s frontend Vega/Vega-Lite runtime will also need to support rendering Vega-Lite v6 specs, since Altair 6 emits a v6.1.0 schema.