fix: Arc mark does not stack for quantitative definitions when different fields are used for theta and radius [draft]

examples/compiled/arc_quantitative_stacked_theta.vg.json

@@ -0,0 +1,76 @@
+{
+  "$schema": "https://vega.github.io/schema/vega/v6.json",
+  "background": "white",
+  "padding": 5,
+  "width": 300,
+  "height": 300,
+  "style": "view",
+  "data": [
+    {
+      "name": "source_0",
+      "values": [
+        {"segment": 1, "value": 8},
+        {"segment": 2, "value": 7},
+        {"segment": 3, "value": 6}
+      ]
+    },
+    {
+      "name": "data_0",
+      "source": "source_0",
+      "transform": [
+        {
+          "type": "stack",
+          "groupby": [],
+          "field": "segment",
+          "sort": {"field": [], "order": []},
+          "as": ["segment_start", "segment_end"],
+          "offset": "zero"
+        },
+        {
+          "type": "filter",
+          "expr": "isValid(datum[\"segment\"]) && isFinite(+datum[\"segment\"]) && isValid(datum[\"value\"]) && isFinite(+datum[\"value\"])"
+        }
+      ]
+    }
+  ],
+  "marks": [
+    {
+      "name": "marks",
+      "type": "arc",
+      "style": ["arc"],
+      "from": {"data": "data_0"},
+      "encode": {
+        "update": {
+          "opacity": {"value": 0.5},
+          "stroke": {"value": "#fff"},
+          "innerRadius": {"value": 20},
+          "fill": {"value": "#4c78a8"},
+          "description": {
+            "signal": "\"segment: \" + (format(datum[\"segment\"], \"\")) + \"; value: \" + (format(datum[\"value\"], \"\"))"
+          },
+          "x": {"signal": "width", "mult": 0.5},
+          "y": {"signal": "height", "mult": 0.5},
+          "outerRadius": {"scale": "radius", "field": "value"},
+          "startAngle": {"scale": "theta", "field": "segment_end"},
+          "endAngle": {"scale": "theta", "field": "segment_start"}
+        }
+      }
+    }
+  ],
+  "scales": [
+    {
+      "name": "theta",
+      "type": "linear",
+      "domain": {"data": "data_0", "fields": ["segment_start", "segment_end"]},
+      "range": [0, 6.283185307179586],
+      "zero": true
+    },
+    {
+      "name": "radius",
+      "type": "sqrt",
+      "domain": {"data": "data_0", "field": "value"},
+      "range": [20, {"signal": "min(width,height)/2"}],
+      "zero": true
+    }
+  ]
+}

examples/specs/arc_quantitative_stacked_theta.vl.json

@@ -0,0 +1,40 @@
+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v6.json",
+  "data": {
+    "values": [
+      {
+        "segment": 1,
+        "value": 8
+      },
+      {
+        "segment": 2,
+        "value": 7
+      },
+      {
+        "segment": 3,
+        "value": 6
+      }
+    ]
+  },
+  "mark": {
+    "type": "arc",
+    "innerRadius": 20,
+    "stroke": "#fff",
+    "opacity": 0.5
+  },
+  "encoding": {
+    "theta": {
+      "field": "segment",
+      "type": "quantitative",
+      "stack": true
+    },
+    "radius": {
+      "field": "value",
+      "scale": {
+        "type": "sqrt",
+        "zero": true,
+        "rangeMin": 20
+      }
+    }
+  }
+}

src/stack.ts

@@ -1,6 +1,6 @@
 import {array, hasOwnProperty, isBoolean} from 'vega-util';
 import {Aggregate, SUM_OPS} from './aggregate.js';
-import {getSecondaryRangeChannel, NonPositionChannel, NONPOSITION_CHANNELS} from './channel.js';
+import {getSecondaryRangeChannel, NonPositionChannel, NONPOSITION_CHANNELS, isPolarPositionChannel} from './channel.js';
 import {
   channelDefType,
   FieldName,
@@ -176,8 +176,13 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
   if (encoding[dimensionChannel]) {
     const dimensionDef = encoding[dimensionChannel];
     const dimensionField = isFieldDef(dimensionDef) ? vgField(dimensionDef, {}) : undefined;
+    const hasSameDimensionAndStackedField = dimensionField && dimensionField === stackedField;
 
-    if (dimensionField && dimensionField !== stackedField) {
+    // For polar coordinates, do not set a groupBy when working with quantitative fields.
+    const isPolar = isPolarPositionChannel(fieldChannel) || isPolarPositionChannel(dimensionChannel);
+    const shouldAddPolarGroupBy = !isUnbinnedQuantitative(dimensionDef);
+
+    if (isPolar ? shouldAddPolarGroupBy : !hasSameDimensionAndStackedField) {
       // avoid grouping by the stacked field
       groupbyChannels.push(dimensionChannel);
       groupbyFields.add(dimensionField);
@@ -237,7 +242,6 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
   if (!offset || !isStackOffset(offset)) {
     return null;
   }
-
   if (isAggregate(encoding) && stackBy.length === 0) {
     return null;
   }

test/stack.test.ts

@@ -448,7 +448,7 @@ describe('stack', () => {
       expect(_stack.stackBy[0].channel).toBe('color');
     });
 
-    it('should be correct for radial chart', () => {
+    it('should be correct for radial chart with same radius and theta quantitative fields', () => {
       const spec: TopLevel<NormalizedUnitSpec> = {
         data: {url: 'data/barley.json'},
         mark: 'arc',
@@ -463,6 +463,27 @@ describe('stack', () => {
       };
       const _stack = stack(spec.mark, spec.encoding);
       expect(_stack.fieldChannel).toBe('theta');
+      expect(_stack.groupbyChannels).toHaveLength(0);
+      expect(_stack.groupbyFields.size).toBe(0);
+    });
+
+    it('should be correct for radial chart with different radius and theta quantitative fields', () => {
+      const spec: TopLevel<NormalizedUnitSpec> = {
+        data: {url: 'data/barley.json'},
+        mark: 'arc',
+        encoding: {
+          theta: {field: 'year', type: 'quantitative', stack: true},
+          radius: {
+            field: 'variety',
+            type: 'quantitative',
+            scale: {type: 'sqrt', zero: true, range: [20, 100]},
+          },
+        },
+      };
+      const _stack = stack(spec.mark, spec.encoding);
+      expect(_stack.fieldChannel).toBe('theta');
+      expect(_stack.groupbyChannels).toHaveLength(0);
+      expect(_stack.groupbyFields.size).toBe(0);
     });
   });