projected links

src/curve.ts

@@ -20,7 +20,14 @@ import {
   curveStepAfter,
   curveStepBefore
 } from "d3";
-import type {CurveFactory, CurveBundleFactory, CurveCardinalFactory, CurveCatmullRomFactory} from "d3";
+import type {
+  CurveFactory,
+  CurveBundleFactory,
+  CurveCardinalFactory,
+  CurveCatmullRomFactory,
+  CurveGenerator,
+  Path
+} from "d3";
 
 type CurveFunction = CurveFactory | CurveBundleFactory | CurveCardinalFactory | CurveCatmullRomFactory;
 type CurveName =
@@ -83,3 +90,16 @@ export function Curve(curve: CurveName | CurveFunction = curveLinear, tension?:
   }
   return c;
 }
+
+// For the “auto” curve, return a symbol instead of a curve implementation;
+// we’ll use d3.geoPath to render if there’s a projection.
+export function PathCurve(curve: CurveName | CurveFunction = curveAuto, tension?: number): CurveFunction {
+  return typeof curve !== "function" && `${curve}`.toLowerCase() === "auto" ? curveAuto : Curve(curve, tension);
+}
+
+// This is a special built-in curve that will use d3.geoPath when there is a
+// projection, and the linear curve when there is not. You can explicitly
+// opt-out of d3.geoPath and instead use d3.line with the "linear" curve.
+export function curveAuto(context: CanvasRenderingContext2D | Path): CurveGenerator {
+  return curveLinear(context);
+}

src/marks/line.js

@@ -1,6 +1,6 @@
-import {curveLinear, geoPath, line as shapeLine} from "d3";
+import {geoPath, line as shapeLine} from "d3";
 import {create} from "../context.js";
-import {Curve} from "../curve.js";
+import {curveAuto, PathCurve} from "../curve.js";
 import {Mark} from "../mark.js";
 import {indexOf, identity, maybeTuple, maybeZ} from "../options.js";
 import {coerceNumbers} from "../scales.js";
@@ -24,22 +24,9 @@ const defaults = {
   strokeMiterlimit: 1
 };
 
-// This is a special built-in curve that will use d3.geoPath when there is a
-// projection, and the linear curve when there is not. You can explicitly
-// opt-out of d3.geoPath and instead use d3.line with the "linear" curve.
-function curveAuto(context) {
-  return curveLinear(context);
-}
-
-// For the “auto” curve, return a symbol instead of a curve implementation;
-// we’ll use d3.geoPath instead of d3.line to render if there’s a projection.
-function LineCurve({curve = curveAuto, tension}) {
-  return typeof curve !== "function" && `${curve}`.toLowerCase() === "auto" ? curveAuto : Curve(curve, tension);
-}
-
 export class Line extends Mark {
   constructor(data, options = {}) {
-    const {x, y, z} = options;
+    const {x, y, z, curve, tension} = options;
     super(
       data,
       {
@@ -51,7 +38,7 @@ export class Line extends Mark {
       defaults
     );
     this.z = z;
-    this.curve = LineCurve(options);
+    this.curve = PathCurve(curve, tension);
     markers(this, options);
   }
   filter(index) {

src/marks/link.js

@@ -1,7 +1,8 @@
-import {pathRound as path} from "d3";
+import {geoPath, pathRound as path} from "d3";
 import {create} from "../context.js";
-import {Curve} from "../curve.js";
+import {curveAuto, PathCurve} from "../curve.js";
 import {Mark} from "../mark.js";
+import {coerceNumbers} from "../scales.js";
 import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyTransform} from "../style.js";
 import {markers, applyMarkers} from "./marker.js";
 
@@ -26,9 +27,15 @@ export class Link extends Mark {
       options,
       defaults
     );
-    this.curve = Curve(curve, tension);
+    this.curve = PathCurve(curve, tension);
     markers(this, options);
   }
+  project(channels, values, context) {
+    // For the auto curve, projection is handled at render.
+    if (this.curve !== curveAuto) {
+      super.project(channels, values, context);
+    }
+  }
   render(index, scales, channels, dimensions, context) {
     const {x1: X1, y1: Y1, x2: X2 = X1, y2: Y2 = Y1} = channels;
     const {curve} = this;
@@ -42,22 +49,43 @@ export class Link extends Mark {
           .enter()
           .append("path")
           .call(applyDirectStyles, this)
-          .attr("d", (i) => {
-            const p = path();
-            const c = curve(p);
-            c.lineStart();
-            c.point(X1[i], Y1[i]);
-            c.point(X2[i], Y2[i]);
-            c.lineEnd();
-            return p;
-          })
+          .attr(
+            "d",
+            curve === curveAuto && context.projection
+              ? sphereLink(context.projection, X1, Y1, X2, Y2)
+              : (i) => {
+                  const p = path();
+                  const c = curve(p);
+                  c.lineStart();
+                  c.point(X1[i], Y1[i]);
+                  c.point(X2[i], Y2[i]);
+                  c.lineEnd();
+                  return p;
+                }
+          )
           .call(applyChannelStyles, this, channels)
           .call(applyMarkers, this, channels)
       )
       .node();
   }
 }
 
+function sphereLink(projection, X1, Y1, X2, Y2) {
+  const path = geoPath(projection);
+  X1 = coerceNumbers(X1);
+  Y1 = coerceNumbers(Y1);
+  X2 = coerceNumbers(X2);
+  Y2 = coerceNumbers(Y2);
+  return (i) =>
+    path({
+      type: "LineString",
+      coordinates: [
+        [X1[i], Y1[i]],
+        [X2[i], Y2[i]]
+      ]
+    });
+}
+
 /** @jsdoc link */
 export function link(data, options = {}) {
   let {x, x1, x2, y, y1, y2, ...remainingOptions} = options;

test/plots/geo-link.js

@@ -0,0 +1,14 @@
+import * as Plot from "@observablehq/plot";
+
+export async function geoLink() {
+  const xy = {x1: [-122.4194], y1: [37.7749], x2: [2.3522], y2: [48.8566]};
+  return Plot.plot({
+    projection: "equal-earth",
+    marks: [
+      Plot.sphere(),
+      Plot.graticule(),
+      Plot.link({length: 1}, {curve: "linear", strokeOpacity: 0.3, ...xy}),
+      Plot.link({length: 1}, {markerEnd: "arrow", ...xy})
+    ]
+  });
+}

test/plots/index.js

@@ -289,6 +289,7 @@ export * from "./electricity-demand.js";
 export * from "./federal-funds.js";
 export * from "./frame.js";
 export * from "./function-contour.js";
+export * from "./geo-link.js";
 export * from "./heatmap.js";
 export * from "./image-rendering.js";
 export * from "./legend-color.js";