fix(writer): boxplot produces invalid Vega-Lite spec

src/writer/vegalite/layer.rs

@@ -2009,9 +2009,8 @@ impl BoxplotRenderer {
         // Build encoding templates for y and y2 fields
         let mut y_encoding = summary_prototype["encoding"][value_var1].clone();
         y_encoding["field"] = json!(value_col);
-        let mut y2_encoding = summary_prototype["encoding"][value_var1].clone();
-        y2_encoding["field"] = json!(value2_col);
-        y2_encoding["title"] = Value::Null; // Suppress y2 title to prevent "y, y2" axis label
+        // y2 is a secondary position channel — Vega-Lite only allows field/datum/value
+        let y2_encoding = json!({"field": value2_col});
 
         // Lower whiskers (rule from y to y2, where y=q1 and y2=lower)
         let mut lower_whiskers = create_layer(

src/writer/vegalite/mod.rs

@@ -296,9 +296,9 @@ fn build_layer_encoding(
             continue;
         }
 
-        // Skip geometry aesthetic - it is structural (consumed by SpatialRenderer
-        // to build GeoJSON Features), not a visual encoding channel.
-        if aesthetic == "geometry" {
+        // Skip structural aesthetics that are consumed by renderers during data
+        // preparation but are not visual encoding channels in Vega-Lite.
+        if aesthetic == "geometry" || aesthetic == "type" {
             continue;
         }
 
@@ -3105,4 +3105,28 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    #[cfg(feature = "duckdb")]
+    fn test_boxplot_schema_validation() {
+        use crate::reader::{DuckDBReader, Reader};
+
+        let reader = DuckDBReader::from_connection_string("duckdb://memory").unwrap();
+        reader
+            .execute_sql(
+                "CREATE TABLE box_data AS
+                SELECT 'A' AS grp, generate_series * 1.0 AS value FROM GENERATE_SERIES(1, 10)
+                UNION ALL
+                SELECT 'B' AS grp, generate_series * 1.0 + 4.0 AS value FROM GENERATE_SERIES(1, 10)",
+            )
+            .unwrap();
+
+        let spec = reader
+            .execute("SELECT * FROM box_data VISUALISE grp AS x, value AS y DRAW boxplot")
+            .unwrap();
+
+        let writer = VegaLiteWriter::new();
+        let json_str = writer.render(&spec).unwrap();
+        assert_valid_vegalite(&json_str);
+    }
 }