dodge declares output channel (#930)

Author: mbostock

src/options.js

@@ -196,6 +196,13 @@ export function maybeColumn(source) {
   return source == null ? [source] : column(source);
 }
 
+// Used by initializers to declare channels that will be initialized. The
+// initial value of these channels is undefined (since they aren’t constructed
+// until the initializer is invoked).
+export function initialized() {
+  return {transform() {}};
+}
+
 export function labelof(value, defaultValue) {
   return typeof value === "string" ? value
     : value && value.label !== undefined ? value.label

src/transforms/dodge.js

@@ -1,6 +1,6 @@
 import IntervalTree from "interval-tree-1d";
 import {finite, positive} from "../defined.js";
-import {identity, number, valueof} from "../options.js";
+import {identity, initialized, number, valueof} from "../options.js";
 import {coerceNumbers} from "../scales.js";
 import {initializer} from "./basic.js";
 
@@ -51,7 +51,7 @@ function dodge(y, x, anchor, padding, options) {
     options = {...options, channels: [...channels ?? [], {name: "r", value: r, scale: "r"}]};
     if (sort === undefined && reverse === undefined) options.sort = {channel: "r", order: "descending"};
   }
-  return initializer(options, function(data, facets, {[x]: X, r: R}, scales, dimensions) {
+  return initializer({...options, [y]: initialized()}, function(data, facets, {[x]: X, r: R}, scales, dimensions) {
     if (!X) throw new Error(`missing channel: ${x}`);
     X = coerceNumbers(valueof(X.value, scales[X.scale] || identity));
     const r = R ? undefined : this.r !== undefined ? this.r : options.r !== undefined ? number(options.r) : 3;

test/output/penguinDodgeVoronoi.svg

@@ -129,6 +129,7 @@ export {default as penguinCulmenVoronoi} from "./penguin-culmen-voronoi.js";
 export {default as penguinVoronoi1D} from "./penguin-voronoi-1d.js";
 export {default as penguinDodge} from "./penguin-dodge.js";
 export {default as penguinDodgeHexbin} from "./penguin-dodge-hexbin.js";
+export {default as penguinDodgeVoronoi} from "./penguin-dodge-voronoi.js";
 export {default as penguinFacetDodge} from "./penguin-facet-dodge.js";
 export {default as penguinFacetDodgeIdentity} from "./penguin-facet-dodge-identity.js";
 export {default as penguinFacetDodgeIsland} from "./penguin-facet-dodge-island.js";

test/plots/index.js

@@ -0,0 +1,13 @@
+import * as Plot from "@observablehq/plot";
+import * as d3 from "d3";
+
+export default async function() {
+  const penguins = await d3.csv("data/penguins.csv", d3.autoType);
+  return Plot.plot({
+    height: 200,
+    marks: [
+      Plot.voronoiMesh(penguins, Plot.dodgeY({x: "body_mass_g"})),
+      Plot.dot(penguins, Plot.dodgeY({x: "body_mass_g", fill: "currentColor"}))
+    ]
+  });
+}