feat: add explicit option to control how densities are resolved, change how densities are resolved by default

build/vega-lite-schema.json

@@ -8143,6 +8143,14 @@
           "description": "The minimum number of samples to take along the extent domain for plotting the density.\n\n__Default value:__ `25`",
           "type": "number"
         },
+        "resolve": {
+          "description": "Indicates how parameters for multiple densities should be resolved. If `\"independent\"`, each density may have its own domain extent and dynamic number of curve sample steps. If `\"shared\"`, the KDE transform will ensure that all densities are defined over a shared domain and curve steps, enabling stacking.\n\n__Default value:__ `\"shared\"`",
+          "enum": [
+            "independent",
+            "shared"
+          ],
+          "type": "string"
+        },
         "steps": {
           "description": "The exact number of samples to take along the extent domain for plotting the density. If specified, overrides both minsteps and maxsteps to set an exact number of uniform samples. Potentially useful in conjunction with a fixed extent to ensure consistent sample points for stacked densities.",
           "type": "number"

examples/compiled/area_density.vg.json

@@ -15,7 +15,8 @@
           "type": "kde",
           "field": "IMDB Rating",
           "bandwidth": 0.3,
-          "as": ["value", "density"]
+          "as": ["value", "density"],
+          "resolve": "shared"
         },
         {
           "type": "impute",

src/compile/data/density.ts

@@ -19,6 +19,8 @@ export class DensityTransformNode extends DataFlowNode {
     this.transform = duplicate(transform); // duplicate to prevent side effects
     const specifiedAs = this.transform.as ?? [undefined, undefined];
     this.transform.as = [specifiedAs[0] ?? 'value', specifiedAs[1] ?? 'density'];
+    const resolve = this.transform.resolve ?? 'shared';
+    this.transform.resolve = resolve;
   }
 
   public dependentFields() {
@@ -40,9 +42,7 @@ export class DensityTransformNode extends DataFlowNode {
       field: density,
       ...rest
     };
-    if (this.transform.groupby) {
-      result.resolve = 'shared';
-    }
+    result.resolve = this.transform.resolve;
     return result;
   }
 }

src/transform.ts

@@ -496,6 +496,14 @@ export interface DensityTransform {
    * __Default value:__ `["value", "density"]`
    */
   as?: [FieldName, FieldName];
+  /**
+   * Indicates how parameters for multiple densities should be resolved.
+   * If `"independent"`, each density may have its own domain extent and dynamic number of curve sample steps.
+   * If `"shared"`, the KDE transform will ensure that all densities are defined over a shared domain and curve steps, enabling stacking.
+   *
+   * __Default value:__ `"shared"`
+   */
+  resolve?: 'independent' | 'shared';
 }
 
 export function isDensity(t: Transform): t is DensityTransform {

test/compile/data/density.test.ts

@@ -41,6 +41,7 @@ describe('compile/data/fold', () => {
       expect(density.assemble()).toEqual({
         type: 'kde',
         field: 'v',
+        resolve: 'shared',
         as: ['value', 'density']
       });
     });
@@ -54,14 +55,16 @@ describe('compile/data/fold', () => {
       expect(density.assemble()).toEqual({
         type: 'kde',
         field: 'v',
+        resolve: 'shared',
         as: ['A', 'density']
       });
     });
 
-    it('should add resolve shared if we group', () => {
+    it('should only add resolve "shared" if we set it explicitly', () => {
       const transform: Transform = {
         density: 'v',
-        groupby: ['a']
+        groupby: ['a'],
+        resolve: 'shared'
       };
       const density = new DensityTransformNode(null, transform);
       expect(density.assemble()).toEqual({
@@ -119,7 +122,7 @@ describe('compile/data/fold', () => {
         as: ['A', 'B']
       };
       const density = new DensityTransformNode(null, transform);
-      expect(density.hash()).toBe('DensityTransform {"as":["A","B"],"density":"v"}');
+      expect(density.hash()).toBe('DensityTransform {"as":["A","B"],"density":"v","resolve":"shared"}');
     });
   });