boolean greys (#694)

Author: mbostock

src/scales/ordinal.js

@@ -4,7 +4,7 @@ import {ascendingDefined} from "../defined.js";
 import {maybeSymbol} from "../options.js";
 import {none} from "../style.js";
 import {registry, color, symbol} from "./index.js";
-import {ordinalScheme, quantitativeScheme} from "./schemes.js";
+import {maybeBooleanRange, ordinalScheme, quantitativeScheme} from "./schemes.js";
 
 export function ScaleO(scale, channels, {
   type,
@@ -26,26 +26,34 @@ export function ScaleO(scale, channels, {
 
 export function ScaleOrdinal(key, channels, {
   type,
+  domain = inferDomain(channels),
   range,
-  scheme = range === undefined ? type === "ordinal" ? "turbo" : "tableau10" : undefined,
+  scheme,
   unknown,
   ...options
 }) {
   let hint;
   if (registry.get(key) === symbol) {
     hint = inferSymbolHint(channels);
     range = range === undefined ? inferSymbolRange(hint) : Array.from(range, maybeSymbol);
-  } else if (registry.get(key) === color && scheme !== undefined) {
-    if (range !== undefined) {
-      const interpolate = quantitativeScheme(scheme);
-      const t0 = range[0], d = range[1] - range[0];
-      range = ({length: n}) => quantize(t => interpolate(t0 + d * t), n);
-    } else {
-      range = ordinalScheme(scheme);
+  } else if (registry.get(key) === color) {
+    if (scheme === undefined
+        && range === undefined
+        && (range = maybeBooleanRange(domain, "greys")) === undefined) {
+      scheme = type === "ordinal" ? "turbo" : "tableau10";
+    }
+    if (scheme !== undefined) {
+      if (range !== undefined) {
+        const interpolate = quantitativeScheme(scheme);
+        const t0 = range[0], d = range[1] - range[0];
+        range = ({length: n}) => quantize(t => interpolate(t0 + d * t), n);
+      } else {
+        range = ordinalScheme(scheme);
+      }
     }
   }
   if (unknown === scaleImplicit) throw new Error("implicit unknown is not supported");
-  return ScaleO(scaleOrdinal().unknown(unknown), channels, {...options, type, range, hint});
+  return ScaleO(scaleOrdinal().unknown(unknown), channels, {...options, type, domain, range, hint});
 }
 
 export function ScalePoint(key, channels, {

src/scales/schemes.js

@@ -143,6 +143,8 @@ const ordinalSchemes = new Map([
 
 function scheme9(scheme, interpolate) {
   return ({length: n}) => {
+    if (n === 1) return [scheme[3][1]]; // favor midpoint
+    if (n === 2) return [scheme[3][0], scheme[3][2]]; // favor extrema
     n = Math.max(3, Math.floor(n));
     return n > 9 ? quantize(interpolate, n) : scheme[n];
   };
@@ -184,6 +186,21 @@ export function ordinalRange(scheme, length) {
   return r.length !== length ? r.slice(0, length) : r;
 }
 
+// If the specified domain contains only booleans (ignoring null and undefined),
+// returns a corresponding range where false is mapped to the low color and true
+// is mapped to the high color of the specified scheme.
+export function maybeBooleanRange(domain, scheme) {
+  const range = new Set();
+  const [f, t] = ordinalScheme(scheme)({length: 2});
+  for (const value of domain) {
+    if (value == null) continue;
+    if (value === true) range.add(t);
+    else if (value === false) range.add(f);
+    else return;
+  }
+  return [...range];
+}
+
 const quantitativeSchemes = new Map([
   // diverging
   ["brbg", interpolateBrBG],