feat: decouple Matplotlib render resolution (DPI) from display size

[daizutabi]

📝 Summary

Closes #9124

This PR decouples the internal rendering resolution (DPI) of Matplotlib figures from their display size in the marimo notebook.

The fundamental principle is that DPI should control image quality (sharpness), not the physical display size.

Previously, increasing fig.dpi to achieve a sharper image caused the plot to expand linearly, often breaking the notebook layout. This change ensures that figures maintain a consistent display size based on a 100 DPI reference (Matplotlib's default), regardless of the rendering DPI.

Changes

• Simplified Rendering Logic: Removed the hardcoded "retina" logic (*2 DPI and //2 dimensions).
• Dynamic Scaling: The rendering now respects the figure's actual DPI setting.
• Metadata Normalization: Display dimensions (width/height) sent to the frontend are now scaled by dpi / 100. This ensures that a figure with a given figsize occupies the same space in the notebook even if the DPI is increased for higher quality.
• Enhanced Testing: Added parameterized tests in tests/_output/formatters/test_matplotlib.py to verify that:
  1. The actual PNG pixel count increases with DPI.
  2. The metadata display size remains consistent regardless of DPI.

Verification

The following image demonstrates the fix:

• Top: fig.dpi = 20 (Low resolution, correct display size)
• Middle: fig.dpi = 300 (High resolution, same display size)
• Bottom: savefig.format = "svg" (Consistent behavior with SVG)

As shown, the display size remains stable even when the DPI is changed significantly, allowing users to control image quality without affecting the notebook layout.

📋 Pre-Review Checklist

• 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).
• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• Video or media evidence is provided for any visual changes (optional).

✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Apr 13, 2026 9:46am

[codecov]

Bundle Report

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[Copilot]

Pull request overview

Decouples Matplotlib PNG render resolution (DPI) from the notebook display size so users can increase sharpness without expanding plot layout (closes #9124).

Changes:

• Remove “retina” hardcoding and render PNGs at the figure’s actual DPI.
• Normalize PNG display metadata (width/height) to a fixed 100-DPI reference so on-screen size stays stable.
• Update/expand tests to validate DPI affects pixel resolution while display sizing metadata remains constant.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
marimo/_output/mpl.py	Render PNG at fig.dpi and scale width/height metadata by dpi/100 to keep display size constant.
tests/_output/formatters/test_matplotlib.py	Replace retina-specific assertions with DPI-parameterized checks for pixel resolution and display-size metadata.
frontend/src/components/editor/Output.tsx	Update comment to reflect metadata’s new purpose (constant display size across DPI/PPI).