curve: "geodesic" for Plot.line

closes #1146

Author: Fil

README.md

@@ -2826,6 +2826,7 @@ The following named curve methods are supported:
 * *step* - a piecewise constant function where *y* changes at the midpoint of *x*
 * *step-after* - a piecewise constant function where *y* changes after *x*
 * *step-before* - a piecewise constant function where *x* changes after *y*
+* *geodesic* - a curve for spherical data along the geodesic (available only for the line mark, associated with a spherical projection)
 
 If *curve* is a function, it will be invoked with a given *context* in the same fashion as a [D3 curve factory](https://github.com/d3/d3-shape/blob/main/README.md#custom-curves).
 

src/marks/line.js

@@ -1,4 +1,4 @@
-import {line as shapeLine} from "d3";
+import {geoPath, line as shapeLine} from "d3";
 import {create} from "../context.js";
 import {Curve} from "../curve.js";
 import {indexOf, identity, maybeTuple, maybeZ} from "../options.js";
@@ -72,9 +72,11 @@ export class Line extends Mark {
 
 /** @jsdoc line */
 export function line(data, options = {}) {
-  let {x, y, ...remainingOptions} = options;
+  let {x, y, curve, ...remainingOptions} = options;
   [x, y] = maybeTuple(x, y);
-  return new Line(data, {...remainingOptions, x, y});
+  return curve === "geodesic"
+    ? new LineGeodesic(data, {...remainingOptions, x, y})
+    : new Line(data, {...remainingOptions, curve, x, y});
 }
 
 /** @jsdoc lineX */
@@ -88,3 +90,50 @@ export function lineY(data, options = {}) {
   const {x = indexOf, y = identity, ...remainingOptions} = options;
   return new Line(data, maybeDenseIntervalX({...remainingOptions, x, y}));
 }
+
+export class LineGeodesic extends Mark {
+  constructor(data, options = {}) {
+    const {x, y, z} = options;
+    super(
+      data,
+      {
+        x: {value: x},
+        y: {value: y},
+        z: {value: maybeZ(options), optional: true}
+      },
+      options,
+      defaults
+    );
+    this.z = z;
+    markers(this, options);
+  }
+  filter(index) {
+    return index;
+  }
+  render(index, scales, channels, dimensions, context) {
+    const {x: X, y: Y} = channels;
+    const {projection} = context;
+    if (projection === undefined) throw new Error("A projection is needed for geodesic curves");
+    const path = geoPath(projection);
+    return create("svg:g", context)
+      .call(applyIndirectStyles, this, scales, dimensions, context)
+      .call(applyTransform, this, scales)
+      .call((g) =>
+        g
+          .selectAll()
+          .data(groupIndex(index, [X, Y], this, channels))
+          .enter()
+          .append("path")
+          .call(applyDirectStyles, this)
+          .call(applyGroupedChannelStyles, this, channels)
+          .call(applyGroupedMarkers, this, channels)
+          .attr("d", (I) =>
+            path({
+              type: "LineString",
+              coordinates: Array.from(I, (i) => [+X[i], +Y[i]]).filter(([x, y]) => !isNaN(x + y))
+            })
+          )
+      )
+      .node();
+  }
+}

test/data/README.md

@@ -19,6 +19,9 @@ https://www.flother.is/2017/olympic-games-data/
 ## barley.csv
 http://search.r-project.org/R/library/lattice/html/barley.html
 
+## beagle.csv
+https://observablehq.com/@bmschmidt/data-driven-projections-darwins-world
+
 ## bls-metro-unemployment.csv
 Bureau of Labor Statistics
 https://www.bls.gov/

test/data/beagle.csv

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