Fix #187 - add sequentialQuantile.quantiles.

README.md

@@ -601,6 +601,10 @@ A [sequential scale](#sequential-scales) with a symmetric logarithmic transform,
 
 A [sequential scale](#sequential-scales) using a *p*-quantile transform, analogous to a [quantile scale](#quantile-scales).
 
+<a name="sequentialQuantile_quantiles" href="#sequentialQuantile_quantiles">#</a> <i>sequentialQuantile</i>.<b>quantiles</b>(<i>n</i>) · [Source](https://github.com/d3/d3-scale/blob/master/src/sequentialQuantile.js), [Examples](https://observablehq.com/@d3/sequential-scales)
+
+Returns an array of *n* + 1 quantiles. For example, if *n* = 4, returns an array of five numbers: the minimum value, the first quartile, the median, the third quartile, and the maximum.
+
 ### Diverging Scales
 
 Diverging scales, like [sequential scales](#sequential-scales), are similar to [continuous scales](#continuous-scales) in that they map a continuous, numeric input domain to a continuous output range. However, unlike continuous scales, the output range of a diverging scale is fixed by its interpolator and not configurable. These scales do not expose [invert](#continuous_invert), [range](#continuous_range), [rangeRound](#continuous_rangeRound) and [interpolate](#continuous_interpolate) methods.

src/sequentialQuantile.js

@@ -1,4 +1,4 @@
-import {ascending, bisect} from "d3-array";
+import {ascending, bisect, quantile} from "d3-array";
 import {identity} from "./continuous.js";
 import {initInterpolator} from "./init.js";
 
@@ -26,6 +26,10 @@ export default function sequentialQuantile() {
     return domain.map((d, i) => interpolator(i / (domain.length - 1)));
   };
 
+  scale.quantiles = function(n) {
+    return Array.from({length: n + 1}, (_, i) => quantile(domain, i / n));
+  };
+
   scale.copy = function() {
     return sequentialQuantile(interpolator).domain(domain);
   };

test/sequentialQuantile-test.js

@@ -22,3 +22,9 @@ tape("sequentialQuantile().range() returns the computed range", function(test) {
   test.deepEqual(s.range(), [0 / 4, 1 / 4, 2 / 4, 3 / 4, 4 / 4]);
   test.end();
 });
+
+tape("sequentialQuantile().quantiles(n) computes n + 1 quantiles", function(test) {
+  var s = scale.scaleSequentialQuantile().domain(Array.from({length: 2000}, (_, i) => 2 * i / 1999));
+  test.deepEqual(s.quantiles(4), [0, 0.5, 1, 1.5, 2]);
+  test.end();
+});