altair: set background to transparent for vegafusion if no background is set

.gitignore

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

marimo/_output/formatters/altair_formatters.py

@@ -18,6 +18,7 @@
 from marimo._plugins.ui._impl.charts.altair_transformer import (
     sanitize_nan_infs,
 )
+from marimo._utils.theme import get_current_theme
 
 LOGGER = marimo_logger()
 
@@ -82,6 +83,15 @@ 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 black for dark mode
+                if (
+                    chart._get("background") is alt.Undefined  # type: ignore
+                    and get_current_theme() == "dark"
+                ):
+                    LOGGER.debug("setting background to black")
+                    chart = chart.properties(background="black")
+
                 return (
                     "application/vnd.vega.v5+json",
                     chart_to_json(chart=chart, spec_format="vega"),

tests/_output/formatters/test_altair_formatters.py

@@ -100,6 +100,31 @@ 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)
+
+        with patch(
+            "marimo._output.formatters.altair_formatters.get_current_theme",
+            return_value="dark",
+        ):
+            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"] == "black"
+
+
 @pytest.mark.skipif(not HAS_DEPS, reason="altair not installed")
 def test_altair_formatter_mimebundle():
     AltairFormatter().register()