Add missing quantile.invertExtent.

Like the quantile and threshold scales, the quantile scale represents a lossy
mapping where a span of values in the domain maps to a discrete range value.

d3.js

@@ -7032,6 +7032,10 @@ d3 = function() {
     scale.quantiles = function() {
       return thresholds;
     };
+    scale.invertExtent = function(y) {
+      y = range.indexOf(y);
+      return y < 0 ? [ NaN, NaN ] : [ y > 0 ? thresholds[y - 1] : domain[0], y < thresholds.length ? thresholds[y] : domain[domain.length - 1] ];
+    };
     scale.copy = function() {
       return d3_scale_quantile(domain, range);
     };
@@ -7061,14 +7065,14 @@ d3 = function() {
       range = x;
       return rescale();
     };
-    scale.copy = function() {
-      return d3_scale_quantize(x0, x1, range);
-    };
     scale.invertExtent = function(y) {
       y = range.indexOf(y);
       y = y < 0 ? NaN : y / kx + x0;
       return [ y, y + 1 / kx ];
     };
+    scale.copy = function() {
+      return d3_scale_quantize(x0, x1, range);
+    };
     return rescale();
   }
   d3.scale.threshold = function() {

src/scale/quantile.js

@@ -38,6 +38,14 @@ function d3_scale_quantile(domain, range) {
     return thresholds;
   };
 
+  scale.invertExtent = function(y) {
+    y = range.indexOf(y);
+    return y < 0 ? [NaN, NaN] : [
+      y > 0 ? thresholds[y - 1] : domain[0],
+      y < thresholds.length ? thresholds[y] : domain[domain.length - 1]
+    ];
+  };
+
   scale.copy = function() {
     return d3_scale_quantile(domain, range); // copy on write!
   };

src/scale/quantize.js

@@ -30,15 +30,15 @@ function d3_scale_quantize(x0, x1, range) {
     return rescale();
   };
 
-  scale.copy = function() {
-    return d3_scale_quantize(x0, x1, range); // copy on write
-  };
-
   scale.invertExtent = function(y) {
     y = range.indexOf(y);
     y = y < 0 ? NaN : y / kx + x0;
     return [y, y + 1 / kx];
   };
 
+  scale.copy = function() {
+    return d3_scale_quantize(x0, x1, range); // copy on write
+  };
+
   return rescale();
 }

test/scale/quantile-test.js

@@ -57,6 +57,33 @@ suite.addBatch({
     "returns undefined if the input value is NaN": function(quantile) {
       var x = quantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]).range([0, 1, 2, 3]);
       assert.isUndefined(x(NaN));
+    },
+    "invertExtent": {
+      "maps a value in the range to a domain extent": function(quantile) {
+        var x = quantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]).range([0, 1, 2, 3]);
+        assert.deepEqual(x.invertExtent(0), [3, 7.25]);
+        assert.deepEqual(x.invertExtent(1), [7.25, 9]);
+        assert.deepEqual(x.invertExtent(2), [9, 14.5]);
+        assert.deepEqual(x.invertExtent(3), [14.5, 20]);
+      },
+      "allows arbitrary range values": function(quantile) {
+        var a = {}, b = {}, x = quantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]).range([a, b]);
+        assert.deepEqual(x.invertExtent(a), [3, 9]);
+        assert.deepEqual(x.invertExtent(b), [9, 20]);
+      },
+      "returns [NaN, NaN] when the given value is not in the range": function(quantile) {
+        var x = quantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]);
+        assert.ok(x.invertExtent(-1).every(isNaN));
+        assert.ok(x.invertExtent(.5).every(isNaN));
+        assert.ok(x.invertExtent(2).every(isNaN));
+        assert.ok(x.invertExtent('a').every(isNaN));
+      },
+      "returns the first match if duplicate values exist in the range": function(quantile) {
+        var x = quantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]).range([0, 1, 2, 0]);
+        assert.deepEqual(x.invertExtent(0), [3, 7.25]);
+        assert.deepEqual(x.invertExtent(1), [7.25, 9]);
+        assert.deepEqual(x.invertExtent(2), [9, 14.5]);
+      }
     }
   }
 });