feat: Output Altair SVG charts as base64-encoded images

[daizutabi]

📝 Summary

This pull request modifies the AltairFormatter to output SVG charts as base64-encoded data URIs.

Currently, Altair SVG charts are rendered as raw <svg> elements, which prevents users from easily saving or dragging the chart as an image file. This change encodes the SVG output into a data:image/svg+xml;base64,... URI, which the frontend then renders as an <img> tag.

This improves the user experience by allowing Altair SVG charts to be dragged and dropped or saved as image files directly from the browser.

Note: This change only impacts users who explicitly opt-in with altair.renderers.enable("svg"). There is no change to the default behavior.

Closes #8400

🔍 Description of Changes

• Modified marimo/_output/formatters/altair_formatters.py to base64-encode SVG string outputs and format them as a data URI.
• Updated tests/_output/formatters/test_altair_formatters.py to assert the new data URI format.

Current behavior: Altair SVG as raw <svg> (lacks image context menu)

After this PR: Altair SVG as Base64-encoded <img> (provides image context menu)

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Tests have been added for the changes made.
• Documentation has been updated where applicable, including docstrings for API changes.
• Pull request title is a good summary of the changes - it will be used in the release notes.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Feb 24, 2026 10:36am