Fix last bin.

src/bin.js

@@ -40,15 +40,17 @@ export default function() {
       // last bin will be zero-width. If the default domain is used, and this
       // last threshold is coincident with the maximum input value, we can
       // extend the niced upper bound by one tick to ensure uniform bin widths;
-      // otherwise, we simply remove the last threshold.
-      if (tz[tz.length - 1] === x1) {
-        if (x1 === max && domain === extent) {
+      // otherwise, we simply remove the last threshold. Note that we don’t
+      // coerce values or the domain to numbers, and thus must be careful to
+      // compare order (>=) rather than strict equality (===)!
+      if (tz[tz.length - 1] >= x1) {
+        if (max >= x1 && domain === extent) {
           const step = tickIncrement(x0, x1, tn);
           if (isFinite(step)) {
             if (step > 0) {
               x1 = (Math.floor(x1 / step) + 1) * step;
             } else if (step < 0) {
-              x1 = (Math.ceil(x1 * step) + 1) / step;
+              x1 = (Math.ceil(x1 * -step) + 1) / -step;
             }
           }
         } else {

test/bin-test.js

@@ -153,6 +153,21 @@ tape("bin()() returns bins whose rightmost bin is not too wide", (test) => {
   ]);
 });
 
+tape("bin(data) coerces values to numbers as expected", (test) => {
+  const h = d3.bin().thresholds(10);
+  test.deepEqual(h(["1", "2", "3", "4", "5"]), [
+    bin(["1"], 1, 1.5),
+    bin([], 1.5, 2),
+    bin(["2"], 2, 2.5),
+    bin([], 2.5, 3),
+    bin(["3"], 3, 3.5),
+    bin([], 3.5, 4),
+    bin(["4"], 4, 4.5),
+    bin([], 4.5, 5),
+    bin(["5"], 5, 5.5)
+  ]);
+});
+
 function bin(bin, x0, x1)  {
   bin.x0 = x0;
   bin.x1 = x1;