fix(mcp): default XY chart x-axis to dataset primary datetime column (#39421)

Co-authored-by: codeant-ai-for-open-source[bot] <244253245+codeant-ai-for-open-source[bot]@users.noreply.github.com>

tests/unit_tests/mcp_service/chart/test_chart_utils.py

@@ -826,6 +826,62 @@ class TestMapConfigToFormData:
         with pytest.raises(ValueError, match="Unsupported config type"):
             map_config_to_form_data("invalid_config")  # type: ignore
 
+    @patch(
+        "superset.mcp_service.chart.chart_utils.is_column_truly_temporal",
+        return_value=True,
+    )
+    @patch("superset.daos.dataset.DatasetDAO.find_by_id")
+    def test_map_xy_config_x_none_defaults_to_main_dttm_col(
+        self, mock_find_by_id: Any, mock_is_temporal: Any
+    ) -> None:
+        """When x is None, map_xy_config resolves it from dataset.main_dttm_col."""
+        mock_dataset = MagicMock()
+        mock_dataset.main_dttm_col = "order_date"
+        mock_find_by_id.return_value = mock_dataset
+
+        config = XYChartConfig(
+            chart_type="xy",
+            x=None,
+            y=[ColumnRef(name="revenue", aggregate="SUM")],
+            kind="bar",
+        )
+
+        result = map_xy_config(config, dataset_id=42)
+
+        assert result["x_axis"] == "order_date"
+        mock_find_by_id.assert_called_once_with(42)
+
+    def test_map_xy_config_x_none_no_dataset_id_raises(self) -> None:
+        """When x is None and no dataset_id, raise ValueError."""
+        config = XYChartConfig(
+            chart_type="xy",
+            x=None,
+            y=[ColumnRef(name="revenue", aggregate="SUM")],
+            kind="line",
+        )
+
+        with pytest.raises(ValueError, match="x-axis column is required"):
+            map_xy_config(config, dataset_id=None)
+
+    @patch("superset.daos.dataset.DatasetDAO.find_by_id")
+    def test_map_xy_config_x_none_no_main_dttm_col_raises(
+        self, mock_find_by_id: Any
+    ) -> None:
+        """When x is None and dataset has no main_dttm_col, raise ValueError."""
+        mock_dataset = MagicMock()
+        mock_dataset.main_dttm_col = None
+        mock_find_by_id.return_value = mock_dataset
+
+        config = XYChartConfig(
+            chart_type="xy",
+            x=None,
+            y=[ColumnRef(name="revenue", aggregate="SUM")],
+            kind="line",
+        )
+
+        with pytest.raises(ValueError, match="no primary datetime column"):
+            map_xy_config(config, dataset_id=42)
+
 
 class TestGenerateChartName:
     """Test generate_chart_name function"""