swatches legend requires ordinal scale (#971)

* swatches legend requires ordinal scale

* Update test/legends/legends-test.ts

Co-authored-by: Philippe Rivière <[email protected]>

Co-authored-by: Philippe Rivière <[email protected]>

Author: mbostock

src/legends/swatches.js

@@ -2,6 +2,7 @@ import {create, path} from "d3";
 import {inferFontVariant} from "../axes.js";
 import {maybeAutoTickFormat} from "../axis.js";
 import {isNoneish, maybeColorChannel, maybeNumberChannel} from "../options.js";
+import {isOrdinalScale} from "../scales.js";
 import {applyInlineStyles, impliedString, maybeClassName} from "../style.js";
 
 function maybeScale(scale, key) {
@@ -12,6 +13,7 @@ function maybeScale(scale, key) {
 }
 
 export function legendSwatches(color, options) {
+  if (!isOrdinalScale(color)) throw new Error(`swatches legend requires ordinal color scale (not ${color.type})`);
   return legendItems(
     color,
     options,

test/legends/legends-test.ts

@@ -1,9 +1,13 @@
 import * as Plot from "@observablehq/plot";
 import * as assert from "assert";
 
-it("Plot.legend({color: {type:'identity'}}) returns undefined", () => {
-  const l = Plot.legend({color: {type: "identity"}});
-  assert.strictEqual(l, undefined);
+it(`Plot.legend({color: {type: "identity"}}) returns undefined`, () => {
+  assert.strictEqual(Plot.legend({color: {type: "identity"}}), undefined);
+});
+
+it(`Plot.legend({legend: "swatches", color: {type: "<not-ordinal>"}}) throws an error`, () => {
+  assert.throws(() => Plot.legend({legend: "swatches", color: {type: "linear"}}), /swatches legend requires ordinal color scale \(not linear\)/);
+  assert.throws(() => Plot.legend({legend: "swatches", color: {type: "diverging"}}), /swatches legend requires ordinal color scale \(not diverging\)/);
 });
 
 it("Plot.legend({}) throws an error", () => {