Add threshold scale invertExtent. Related #1201.

d3.js

@@ -6976,6 +6976,10 @@ d3 = function() {
       range = _;
       return scale;
     };
+    scale.invertExtent = function(y) {
+      var i = range.indexOf(y);
+      return [ domain[i - 1], domain[i] ];
+    };
     scale.copy = function() {
       return d3_scale_threshold(domain, range);
     };

src/scale/threshold.js

@@ -23,6 +23,11 @@ function d3_scale_threshold(domain, range) {
     return scale;
   };
 
+  scale.invertExtent = function(y) {
+    var i = range.indexOf(y);
+    return [domain[i - 1], domain[i]];
+  };
+
   scale.copy = function() {
     return d3_scale_threshold(domain, range);
   };

test/scale/threshold-test.js

@@ -42,6 +42,14 @@ suite.addBatch({
       assert.equal(x(.6), b);
       assert.equal(x(.8), c);
       assert.equal(x(1), c);
+    },
+    "invertExtent": {
+      "returns the domain extent for the specified range value": function(threshold) {
+        var a = {}, b = {}, c = {}, x = threshold().domain([1/3, 2/3]).range([a, b, c]);
+        assert.deepEqual(x.invertExtent(a), [undefined, 1/3]);
+        assert.deepEqual(x.invertExtent(b), [1/3, 2/3]);
+        assert.deepEqual(x.invertExtent(c), [2/3, undefined]);
+      }
     }
   }
 });