altair: set background to transparent for vegafusion if no background is set (#6614)

## 📝 Summary

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If this PR fixes any issues, list them here by number (e.g., Fixes
#123).
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Fixes #6601 .

dark:
<img width="749" height="365" alt="CleanShot 2025-10-01 at 02 17 28"
src="https://github.com/user-attachments/assets/35fdf4ca-77cc-40c4-846d-5ac2b4c3960c"
/>

light:
<img width="785" height="574" alt="CleanShot 2025-10-01 at 02 17 04"
src="https://github.com/user-attachments/assets/2160f54a-8029-44ed-8387-4421c9e4eace"
/>


## 🔍 Description of Changes

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problem addressed and how it was resolved. If applicable, provide before
and after comparisons, screenshots, or any relevant details to help
reviewers understand the changes easily.
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## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
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[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [x] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

Author: Light2Dark

.gitignore

@@ -96,6 +96,7 @@ uv.lock
 
 # WIP files
 .wip/
+wip/
 
 # editors
 *.swp

marimo/_output/formatters/altair_formatters.py

@@ -82,6 +82,14 @@ def _show_chart(chart: AltairChartType) -> tuple[KnownMimeType, str]:
 
             # If vegafusion is enabled, just wrap in altair_chart
             if alt.data_transformers.active.startswith("vegafusion"):
+                # Bug https://github.com/marimo-team/marimo/issues/6601. Vegafusion defaults to white background
+                # So, we set the background to transparent
+                if (
+                    chart._get("background") is alt.Undefined  # type: ignore
+                ):
+                    LOGGER.debug("setting background to transparent")
+                    chart = chart.properties(background="transparent")
+
                 return (
                     "application/vnd.vega.v5+json",
                     chart_to_json(chart=chart, spec_format="vega"),

tests/_output/formatters/test_altair_formatters.py

@@ -100,6 +100,27 @@ def test_altair_formatter_full_width(mock_make_full_width: MagicMock):
     mock_make_full_width.assert_called_once()
 
 
+@pytest.mark.skipif(not HAS_DEPS, reason="altair not installed")
+def test_altair_formatter_vegafusion_dark_mode():
+    AltairFormatter().register()
+
+    import altair as alt
+
+    with alt.data_transformers.enable("vegafusion"):
+        chart = alt.Chart(get_data()).mark_point()
+        formatter = get_formatter(chart)
+
+        assert formatter is not None
+        res = formatter(chart)
+        assert res is not None
+        mime, content = res
+        assert mime == "application/vnd.vega.v5+json"
+        assert isinstance(content, str)
+        json_data = json.loads(content)
+        assert "background" in json_data
+        assert json_data["background"] == "transparent"
+
+
 @pytest.mark.skipif(not HAS_DEPS, reason="altair not installed")
 def test_altair_formatter_mimebundle():
     AltairFormatter().register()