Add A2UI heatmap charts

src/a2ui_validation.rs

@@ -23,6 +23,12 @@ const CASES: &[Case] = &[
         components: &["Chart"],
         chart_type: Some("donut"),
     },
+    Case {
+        name: "heatmap-natural",
+        prompt: "Do not call tools. Show a heatmap of pull requests reviewed in 2025 and 2026, with each year as a row and January, February, and March as columns. Use values 2, 5, 3 for 2025 and 4, 1, 6 for 2026.",
+        components: &["Chart"],
+        chart_type: Some("heatmap"),
+    },
     Case {
         name: "form-controls",
         prompt: "Do not call tools. Build an A2UI Form for a name, multiline notes, due date, and multiple checkbox priorities, with a submit button.",