Support ordinal scale inversion.

README.md

@@ -734,6 +734,19 @@ Constructs a new ordinal scale with an empty [domain](#ordinal_domain) and the s
 
 Given a *value* in the input [domain](#ordinal_domain), returns the corresponding value in the output [range](#ordinal_range). If the given *value* is not in the scale’s [domain](#ordinal_domain), returns the [unknown](#ordinal_value); or, if the unknown value is [implicit](#implicit) (the default), then the *value* is implicitly added to the domain and the next-available value in the range is assigned to *value*, such that this and subsequent invocations of the scale given the same input *value* return the same output value.
 
+<a name="ordinal_invert" href="#ordinal_invert">#</a> <i>ordinal</i>.<b>invert</b>(<i>value</i>)
+
+Given a *value* from the [range](#ordinal_range), returns the first corresponding value from the [domain](#ordinal_domain). If the range includes duplicate values, this method returns the corresponding value with the lowest index in the domain array. For example:
+
+```js
+var ordinal = d3.scaleOrdinal()
+    .domain(["a", "b", "c"])
+    .range(["red", "white", "red"]);
+
+ordinal.invert("red"); // "a"
+ordinal.invert("white"); // "b"
+```
+
 <a name="ordinal_domain" href="#ordinal_domain">#</a> <i>ordinal</i>.<b>domain</b>([<i>domain</i>])
 
 If *domain* is specified, sets the domain to the specified array of values. The first element in *domain* will be mapped to the first element in the range, the second domain value to the second range value, and so on. Domain values are stored internally in a map from stringified value to index; the resulting index is then used to retrieve a value from the range. Thus, an ordinal scale’s values must be coercible to a string, and the stringified version of the domain value uniquely identifies the corresponding range value. If *domain* is not specified, this method returns the current domain.

src/band.js

@@ -14,6 +14,7 @@ export default function band() {
       align = 0.5;
 
   delete scale.unknown;
+  delete scale.invert;
 
   function rescale() {
     var n = domain().length,

src/ordinal.js

@@ -5,6 +5,7 @@ export var implicit = {name: "implicit"};
 
 export default function ordinal(range) {
   var index = map(),
+      inverseIndex = null,
       domain = [],
       unknown = implicit;
 
@@ -15,13 +16,14 @@ export default function ordinal(range) {
     if (!i) {
       if (unknown !== implicit) return unknown;
       index.set(key, i = domain.push(d));
+      inverseIndex = null;
     }
     return range[(i - 1) % range.length];
   }
 
   scale.domain = function(_) {
     if (!arguments.length) return domain.slice();
-    domain = [], index = map();
+    domain = [], index = map(), inverseIndex = 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;
@@ -35,6 +37,15 @@ export default function ordinal(range) {
     return arguments.length ? (unknown = _, scale) : unknown;
   };
 
+  scale.invert = function(_) {
+    if (!inverseIndex) {
+      inverseIndex = map();
+      var n = domain.length;
+      while (--n >= 0) inverseIndex.set(range[n], n+1);
+    }
+    return domain[inverseIndex.get(_) - 1];
+  };
+
   scale.copy = function() {
     return ordinal()
         .domain(domain)
@@ -43,4 +54,4 @@ export default function ordinal(range) {
   };
 
   return scale;
-}
+}

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();
+});