From 5b8ee24e60211305a80cf8a9c57c01f0fcaf5dee Mon Sep 17 00:00:00 2001
From: Younghoon Kim <yh.kim@databricks.com>
Date: Wed, 19 Jul 2023 12:54:31 -0700
Subject: [PATCH] Set stack for area with plain Qs on the both axes

---
 examples/specs/area_cumulative_freq.vl.json |  3 ++-
 src/stack.ts                                |  2 +-
 test/stack.test.ts                          | 15 +++++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/examples/specs/area_cumulative_freq.vl.json b/examples/specs/area_cumulative_freq.vl.json
index d74bc5dd933..82a51396bbe 100644
--- a/examples/specs/area_cumulative_freq.vl.json
+++ b/examples/specs/area_cumulative_freq.vl.json
@@ -14,7 +14,8 @@
     },
     "y": {
       "field": "Cumulative Count",
-      "type": "quantitative"
+      "type": "quantitative",
+      "aggregate": "max"
     }
   }
 }
diff --git a/src/stack.ts b/src/stack.ts
index 22fa87ce121..db05480795a 100644
--- a/src/stack.ts
+++ b/src/stack.ts
@@ -106,7 +106,7 @@ function potentialStackedChannel(
         return xAggregate ? x : y;
       }
 
-      if (isCartesian && mark === 'bar') {
+      if (isCartesian && ['bar', 'area'].includes(mark)) {
         if (orient === 'vertical') {
           return y;
         } else if (orient === 'horizontal') {
diff --git a/test/stack.test.ts b/test/stack.test.ts
index f8d29dc23d4..455748d07d9 100644
--- a/test/stack.test.ts
+++ b/test/stack.test.ts
@@ -327,6 +327,21 @@ describe('stack', () => {
       }
     });
 
+    it('should stack even for two plain quantitatives with the orient on the axes', () => {
+      for (const mark of [BAR, AREA]) {
+        const spec: TopLevel<NormalizedUnitSpec> = {
+          data: {url: 'data/movies.json'},
+          mark: {type: mark, orient: 'vertical'}, // orient also can be inferred by init.ts
+          encoding: {
+            x: {field: 'IMDB Rating', type: 'quantitative'},
+            y: {field: 'Rotten Tomatoes Rating', type: 'quantitative'}
+          }
+        };
+        const stackProps = stack(spec.mark, spec.encoding);
+        expect(stackProps.fieldChannel).toBe(Y);
+      }
+    });
+
     it('should be correct for horizontal (single)', () => {
       for (const stackableMark of [BAR, AREA]) {
         const spec: TopLevel<NormalizedUnitSpec> = {