fix: allow adding groupby channel when spec has a color encoding

Author: hydrosquall

src/stack.ts

@@ -179,13 +179,14 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
     const hasSameDimensionAndStackedField = dimensionField && dimensionField === stackedField;
 
     // This change breaks cartesian tests, just pushing to start the discussion. When dev'ing locally, try removing this condition.
+    // 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)) {
+    if (!hasSameDimensionAndStackedField && (isPolar ? dimensionChannel === fieldChannel || encoding['color'] : true)) {
       // avoid grouping by the stacked field
       // TKTK: find out why
       groupbyChannels.push(dimensionChannel);
-      groupbyFields.add(dimensionField); // Iinvestigate meee
+      groupbyFields.add(dimensionField);
     }
   }