delaunay X and Y are optional (#931)

* delaunay X and Y are optional

A complement to observablehq/plot#930

the X and Y channels should be optional in all the voronoi marks—since d3-delaunay can handle collinear points (and it's not just for the sake of it: the resulting voronoi can be useful, as illustrated in the test case).

* adopt applyFrameAnchor

* one fewer closure

Co-authored-by: Mike Bostock <[email protected]>

Author: tigrevol8888

src/marks/delaunay.js

@@ -1,8 +1,8 @@
 import {create, group, path, select, Delaunay} from "d3";
 import {Curve} from "../curve.js";
-import {maybeTuple, maybeZ} from "../options.js";
+import {constant, maybeTuple, maybeZ} from "../options.js";
 import {Mark} from "../plot.js";
-import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyTransform, offset} from "../style.js";
+import {applyChannelStyles, applyDirectStyles, applyFrameAnchor, applyIndirectStyles, applyTransform, offset} from "../style.js";
 import {markers, applyMarkers} from "./marker.js";
 
 const delaunayLinkDefaults = {
@@ -47,8 +47,8 @@ class DelaunayLink extends Mark {
     super(
       data,
       [
-        {name: "x", value: x, scale: "x"},
-        {name: "y", value: y, scale: "y"},
+        {name: "x", value: x, scale: "x", optional: true},
+        {name: "y", value: y, scale: "y", optional: true},
         {name: "z", value: z, optional: true}
       ],
       options,
@@ -60,6 +60,9 @@ class DelaunayLink extends Mark {
   render(index, {x, y}, channels, dimensions) {
     const {x: X, y: Y, z: Z} = channels;
     const {dx, dy, curve} = this;
+    const [cx, cy] = applyFrameAnchor(this, dimensions);
+    const xi = X ? i => X[i] : constant(cx);
+    const yi = Y ? i => Y[i] : constant(cy);
     const mark = this;
 
     function links(index) {
@@ -76,14 +79,14 @@ class DelaunayLink extends Mark {
         ti = index[ti];
         tj = index[tj];
         newIndex.push(++i);
-        X1[i] = X[ti];
-        Y1[i] = Y[ti];
-        X2[i] = X[tj];
-        Y2[i] = Y[tj];
+        X1[i] = xi(ti);
+        Y1[i] = yi(ti);
+        X2[i] = xi(tj);
+        Y2[i] = yi(tj);
         for (const k in channels) newChannels[k].push(channels[k][tj]);
       }
 
-      const {halfedges, hull, triangles} = Delaunay.from(index, i => X[i], i => Y[i]);
+      const {halfedges, hull, triangles} = Delaunay.from(index, xi, yi);
       for (let i = 0; i < halfedges.length; ++i) { // inner edges
         const j = halfedges[i];
         if (j > i) link(triangles[i], triangles[j]);
@@ -126,33 +129,37 @@ class AbstractDelaunayMark extends Mark {
     super(
       data,
       [
-        {name: "x", value: x, scale: "x"},
-        {name: "y", value: y, scale: "y"},
+        {name: "x", value: x, scale: "x", optional: true},
+        {name: "y", value: y, scale: "y", optional: true},
         {name: "z", value: zof(options), optional: true}
       ],
       options,
       defaults
     );
   }
-  render(index, {x, y}, {x: X, y: Y, z: Z, ...channels}, dimensions) {
+  render(index, {x, y}, channels, dimensions) {
+    const {x: X, y: Y, z: Z} = channels;
     const {dx, dy} = this;
+    const [cx, cy] = applyFrameAnchor(this, dimensions);
+    const xi = X ? i => X[i] : constant(cx);
+    const yi = Y ? i => Y[i] : constant(cy);
     const mark = this;
-    function mesh(render) {
-      return function(index) {
-        const delaunay = Delaunay.from(index, i => X[i], i => Y[i]);
-        select(this).append("path")
-          .datum(index[0])
-          .call(applyDirectStyles, mark)
-          .attr("d", render(delaunay, dimensions))
-          .call(applyChannelStyles, mark, channels);
-      };
+
+    function mesh(index) {
+      const delaunay = Delaunay.from(index, xi, yi);
+      select(this).append("path")
+        .datum(index[0])
+        .call(applyDirectStyles, mark)
+        .attr("d", mark._render(delaunay, dimensions))
+        .call(applyChannelStyles, mark, channels);
     }
+
     return create("svg:g")
         .call(applyIndirectStyles, this, dimensions)
         .call(applyTransform, x, y, offset + dx, offset + dy)
         .call(Z
-          ? g => g.selectAll().data(group(index, i => Z[i]).values()).enter().append("g").each(mesh(this._render))
-          : g => g.datum(index).each(mesh(this._render)))
+          ? g => g.selectAll().data(group(index, i => Z[i]).values()).enter().append("g").each(mesh)
+          : g => g.datum(index).each(mesh))
       .node();
   }
 }
@@ -182,8 +189,8 @@ class Voronoi extends Mark {
     super(
       data,
       [
-        {name: "x", value: x, scale: "x"},
-        {name: "y", value: y, scale: "y"},
+        {name: "x", value: x, scale: "x", optional: true},
+        {name: "y", value: y, scale: "y", optional: true},
         {name: "z", value: z, optional: true}
       ],
       options,
@@ -193,9 +200,12 @@ class Voronoi extends Mark {
   render(index, {x, y}, channels, dimensions) {
     const {x: X, y: Y, z: Z} = channels;
     const {dx, dy} = this;
+    const [cx, cy] = applyFrameAnchor(this, dimensions);
+    const xi = X ? i => X[i] : constant(cx);
+    const yi = Y ? i => Y[i] : constant(cy);
 
     function cells(index) {
-      const delaunay = Delaunay.from(index, i => X[i], i => Y[i]);
+      const delaunay = Delaunay.from(index, xi, yi);
       const voronoi = voronoiof(delaunay, dimensions);
       select(this)
         .selectAll()