Add rotation.invert.

Fixes #1134.

d3.js

@@ -6774,10 +6774,15 @@ d3 = function() {
   }
   d3.geo.rotation = function(rotate) {
     rotate = d3_geo_rotation(rotate[0] % 360 * d3_radians, rotate[1] * d3_radians, rotate.length > 2 ? rotate[2] * d3_radians : 0);
-    return function(coordinates) {
+    function forward(coordinates) {
       coordinates = rotate(coordinates[0] * d3_radians, coordinates[1] * d3_radians);
       return coordinates[0] *= d3_degrees, coordinates[1] *= d3_degrees, coordinates;
+    }
+    forward.invert = function(coordinates) {
+      coordinates = rotate.invert(coordinates[0] * d3_radians, coordinates[1] * d3_radians);
+      return coordinates[0] *= d3_degrees, coordinates[1] *= d3_degrees, coordinates;
     };
+    return forward;
   };
   function d3_geo_rotation(δλ, δφ, δγ) {
     return δλ ? δφ || δγ ? d3_geo_compose(d3_geo_rotationλ(δλ), d3_geo_rotationφγ(δφ, δγ)) : d3_geo_rotationλ(δλ) : δφ || δγ ? d3_geo_rotationφγ(δφ, δγ) : d3_geo_equirectangular;

src/geo/rotation.js

@@ -1,9 +1,17 @@
 d3.geo.rotation = function(rotate) {
   rotate = d3_geo_rotation(rotate[0] % 360 * d3_radians, rotate[1] * d3_radians, rotate.length > 2 ? rotate[2] * d3_radians : 0);
-  return function(coordinates) {
+
+  function forward(coordinates) {
     coordinates = rotate(coordinates[0] * d3_radians, coordinates[1] * d3_radians);
     return coordinates[0] *= d3_degrees, coordinates[1] *= d3_degrees, coordinates;
+  }
+
+  forward.invert = function(coordinates) {
+    coordinates = rotate.invert(coordinates[0] * d3_radians, coordinates[1] * d3_radians);
+    return coordinates[0] *= d3_degrees, coordinates[1] *= d3_degrees, coordinates;
   };
+
+  return forward;
 };
 
 // Note: |δλ| must be < 2π

test/geo/rotation-test.js

@@ -23,6 +23,9 @@ suite.addBatch({
     },
     "rotates longitude and latitude": function(rotation) {
       assert.inDelta(rotation([0, 0]), [-54.73561, 30], 1e-6);
+    },
+    "inverse rotation of longitude and latitude": function(rotation) {
+      assert.inDelta(rotation.invert([-54.73561, 30]), [0, 0], 1e-6);
     }
   }
 });