Add ordinal invert.

src/ordinal.js

@@ -5,6 +5,7 @@ export var implicit = {name: "implicit"};
 
 export default function ordinal() {
   var index = map(),
+      invIndex = null,
       domain = [],
       range = [],
       unknown = implicit;
@@ -14,13 +15,14 @@ export default function ordinal() {
     if (!i) {
       if (unknown !== implicit) return unknown;
       index.set(key, i = domain.push(d));
+      invIndex = null;
     }
     return range[(i - 1) % range.length];
   }
 
   scale.domain = function(_) {
     if (!arguments.length) return domain.slice();
-    domain = [], index = map();
+    domain = [], index = map(), invIndex = null;
     var i = -1, n = _.length, d, key;
     while (++i < n) if (!index.has(key = (d = _[i]) + "")) index.set(key, domain.push(d));
     return scale;
@@ -34,6 +36,15 @@ export default function ordinal() {
     return arguments.length ? (unknown = _, scale) : unknown;
   };
 
+  scale.invert = function(_) {
+    if (!invIndex) {
+      invIndex = map();
+      var n = domain.length;
+      while (--n >= 0) invIndex.set(range[n], n+1);
+    }
+    return domain[invIndex.get(_ + "") - 1];
+  };
+
   scale.copy = function() {
     return ordinal()
         .domain(domain)

test/ordinal-test.js

@@ -202,3 +202,13 @@ tape("ordinal.copy() changes to the range are isolated", function(test) {
   test.deepEqual(s2.range(), ["foo", "baz"]);
   test.end();
 });
+
+tape("ordinal.invert(x) returns first matching domain value", function(test) {
+  var s = scale.scaleOrdinal().domain(["foo", "bar", "baz", "oof"]).range(["a", "b", "", "a"]);
+  test.equal(s.invert(), undefined);
+  test.equal(s.invert("a"), "foo");
+  test.equal(s.invert("b"), "bar");
+  test.equal(s.invert(""), "baz");
+  test.equal(s.invert("c"), undefined);
+  test.end();
+});