add / explain special case for area marks

Author: hydrosquall

.cursorignore

@@ -0,0 +1,2 @@
+docs/*.js
+node_modules/# Add directories or file patterns to ignore during indexing (e.g. foo/ or *.csv)

src/stack.ts

@@ -177,25 +177,25 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<string>): StackPrope
     const dimensionDef = encoding[dimensionChannel];
     const dimensionField = isFieldDef(dimensionDef) ? vgField(dimensionDef, {}) : undefined;
     const hasSameDimensionAndStackedField = dimensionField && dimensionField === stackedField;
-    const isUnbinnedQuant = isUnbinnedQuantitative(dimensionDef);
+    const isDimensionUnbinnedQuant = isUnbinnedQuantitative(dimensionDef);
 
     // Check if both the field channel and dimension channel have unbinned quantitative fields
-    const isFieldUnbinnedQuant = isUnbinnedQuantitative(stackedFieldDef);
+    const isStackedFieldUnbinnedQuant = isUnbinnedQuantitative(stackedFieldDef);
 
-    const bothChannelsAreUnbinnedQuantitative = isUnbinnedQuant && isFieldUnbinnedQuant;
+    // Both channels are unbinned quantitative if both the dimension and field channels are unbinned quantitative
+    const bothChannelsAreUnbinnedQuantitative = isDimensionUnbinnedQuant && isStackedFieldUnbinnedQuant;
 
-    // For area charts, we need to ensure we still group by the dimension field
-    // even if both fields are quantitative (e.g., for cumulative frequency plots)
+    // Special case for area charts: they need dimension fields in groupBy even when both fields are quantitative
+    // Example use: visualizations like cumulative frequency plots where the area needs to be properly filled
+    // Ref STACK_BY_DEFAULT_MARKS and STACKABLE_MARKS
     const isAreaMark = mark === 'area';
 
     // The core logic: we only add to groupBy when:
     // 1. The dimension field is different from the stacked field (to avoid redundant grouping)
     // 2. AND either:
-    //    a. The dimension field is not an unbinned quantitative field, OR
-    //    b. The field channel is not an unbinned quantitative field, OR
-    //    c. It's an area mark (which needs grouping even with quantitative fields)
-    const shouldAddToGroupBy =
-      !hasSameDimensionAndStackedField && (!isUnbinnedQuant || !bothChannelsAreUnbinnedQuantitative || isAreaMark);
+    //    a. Not both channels are unbinned quantitative, OR
+    //    b. It's an area mark (which needs grouping even with quantitative fields)
+    const shouldAddToGroupBy = !hasSameDimensionAndStackedField && (!bothChannelsAreUnbinnedQuantitative || isAreaMark);
 
     if (shouldAddToGroupBy) {
       // avoid grouping by the stacked field