Prefer interpolateNumber when coercible to number.

Objects that are coercible to numbers, such as Dates, are often more usefully
interpolated as numbers rather than as objects.

d3.js

@@ -5692,7 +5692,7 @@
   }
   d3.interpolators = [ function(a, b) {
     var t = typeof b;
-    return (t === "string" ? d3_rgb_names.has(b) || /^(#|rgb\(|hsl\()/.test(b) ? d3_interpolateRgb : d3_interpolateString : b instanceof d3_Color ? d3_interpolateRgb : t === "object" ? Array.isArray(b) ? d3_interpolateArray : d3_interpolateObject : d3_interpolateNumber)(a, b);
+    return (t === "string" ? d3_rgb_names.has(b) || /^(#|rgb\(|hsl\()/.test(b) ? d3_interpolateRgb : d3_interpolateString : b instanceof d3_Color ? d3_interpolateRgb : Array.isArray(b) ? d3_interpolateArray : t === "object" && isNaN(b) ? d3_interpolateObject : d3_interpolateNumber)(a, b);
   } ];
   d3.interpolateArray = d3_interpolateArray;
   function d3_interpolateArray(a, b) {

src/interpolate/interpolate.js

@@ -19,7 +19,8 @@ d3.interpolators = [
     var t = typeof b;
     return (t === "string" ? (d3_rgb_names.has(b) || /^(#|rgb\(|hsl\()/.test(b) ? d3_interpolateRgb : d3_interpolateString)
         : b instanceof d3_Color ? d3_interpolateRgb
-        : t === "object" ? (Array.isArray(b) ? d3_interpolateArray : d3_interpolateObject)
+        : Array.isArray(b) ? d3_interpolateArray
+        : t === "object" && isNaN(b) ? d3_interpolateObject
         : d3_interpolateNumber)(a, b);
   }
 ];

test/interpolate/interpolate-test.js

@@ -95,11 +95,11 @@ suite.addBatch({
       "interpolates each property in b": function(d3) {
         assert.deepEqual(d3.interpolate({foo: 2, bar: 4}, {foo: 12, bar: 24})(.4), {foo: 6, bar: 12});
       },
-      "interpolates arrays, even when both a and b are coercible to numbers": function(d3) {
+      "interpolates numbers if b is coercible to a number (not NaN)": function(d3) {
         var two = new Number(2), twelve = new Number(12);
-        two.foo = "2px";
-        twelve.foo = "12px";
-        assert.deepEqual(d3.interpolate(two, twelve)(.4), {foo: "6px"});
+        assert.strictEqual(d3.interpolate(two, twelve)(.4), 6);
+        var start = new Date(2012, 0, 1), end = new Date(2013, 0, 1);
+        assert.strictEqual(d3.interpolate(start, end)(.5), +new Date(2012, 6, 2, 1));
       },
       "reuses the returned object during interpolation": function(d3) {
         var i = d3.interpolate({foo: 2, bar: 4}, {foo: 12, bar: 24});