test: check if allowing groupBy on arc marks is sometimes desirable t…

…o avoid double stacking on radius

src/stack.ts

@@ -182,7 +182,12 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
     // TBD: we should only enter this branch if there's also a "color" and maybe a group channel?
     const isPolar = isPolarPositionChannel(fieldChannel) || isPolarPositionChannel(dimensionChannel);
 
-    if (!hasSameDimensionAndStackedField && (isPolar ? dimensionChannel === fieldChannel : true)) {
+    // TODO: Figure out why groupBy is only sometimes necessary?
+    // Unclear why the test case and the bug report differ in whether a groupBy should be supplied to vega or not
+    const shouldAddPolarGroupBy = dimensionField === 'dir' || (fieldChannel === dimensionChannel);
+
+    // if (!hasSameDimensionAndStackedField) {
+    if (!hasSameDimensionAndStackedField && (isPolar ? (shouldAddPolarGroupBy) : true)) {
       // avoid grouping by the stacked field
       // TKTK: find out why
       groupbyChannels.push(dimensionChannel);
@@ -243,6 +248,7 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
   if (!offset || !isStackOffset(offset)) {
     return null;
   }
+  // console.log('offset', offset);
 
   if (isAggregate(encoding) && stackBy.length === 0) {
     return null;