Fix boundary case when filtering histogram thresholds.

Also document that thresholds outside the domain are ignored.

Fixes #54.

Author: mbostock

README.md

@@ -357,6 +357,8 @@ Note that the domain accessor is invoked on the materialized array of [values](#
 
 If *thresholds* is specified, sets the [threshold generator](#histogram-thresholds) to the specified function or array and returns this histogram generator. If *thresholds* is not specified, returns the current threshold generator, which by default implements [Sturges’ formula](#thresholdSturges). (Thus by default, the histogram values must be numbers!) Thresholds are defined as an array of values [*x0*, *x1*, …]. Any value less than *x0* will be placed in the first bin; any value greater than or equal to *x0* but less than *x1* will be placed in the second bin; and so on. Thus, the [generated histogram](#_histogram) will have *thresholds*.length + 1 bins. See [histogram thresholds](#histogram-thresholds) for more information.
 
+Any threshold values outside the [domain](#histogram_domain) are ignored. The first *bin*.x0 is always equal to the minimum domain value, and the last *bin*.x1 is always equal to the maximum domain value.
+
 If a *count* is specified instead of an array of *thresholds*, then the [domain](#histogram_domain) will be uniformly divided into approximately *count* bins; see [ticks](#ticks).
 
 ### Histogram Thresholds

src/histogram.js

@@ -3,7 +3,8 @@ import bisect from "./bisect";
 import constant from "./constant";
 import extent from "./extent";
 import identity from "./identity";
-import ticks from "./ticks";
+import range from "./range";
+import {tickStep} from "./ticks";
 import sturges from "./threshold/sturges";
 
 export default function() {
@@ -27,12 +28,15 @@ export default function() {
         tz = threshold(values, x0, x1);
 
     // Convert number of thresholds into uniform thresholds.
-    if (!Array.isArray(tz)) tz = ticks(x0, x1, tz);
+    if (!Array.isArray(tz)) {
+      tz = tickStep(x0, x1, tz);
+      tz = range(Math.ceil(x0 / tz) * tz, Math.floor(x1 / tz) * tz, tz); // exclusive
+    }
 
     // Remove any thresholds outside the domain.
     var m = tz.length;
     while (tz[0] <= x0) tz.shift(), --m;
-    while (tz[m - 1] >= x1) tz.pop(), --m;
+    while (tz[m - 1] > x1) tz.pop(), --m;
 
     var bins = new Array(m + 1),
         bin;

test/histogram-test.js

@@ -90,6 +90,17 @@ tape("histogram.thresholds(array) sets the bin thresholds", function(test) {
   test.end();
 });
 
+tape("histogram.thresholds(array) ignores thresholds outside the domain", function(test) {
+  var h = arrays.histogram().thresholds([0, 1, 2, 3]);
+  test.deepEqual(h([0, 1, 2, 3]), [
+    bin([0], 0, 1),
+    bin([1], 1, 2),
+    bin([2], 2, 3),
+    bin([3], 3, 3) // Note: inclusive upper bound for last bin.
+  ]);
+  test.end();
+});
+
 tape("histogram.thresholds(function) sets the bin thresholds accessor", function(test) {
   var actual,
       values = [0, 0, 0, 10, 30, 30],