Allow projection.invert to return null.

d3.js

@@ -6072,7 +6072,7 @@
       return x = a(x, y), b(x[0], x[1]);
     }
     if (a.invert && b.invert) compose.invert = function(x, y) {
-      return x = b.invert(x, y), a.invert(x[0], x[1]);
+      return x = b.invert(x, y), x && a.invert(x[0], x[1]);
     };
     return compose;
   }
@@ -6514,7 +6514,7 @@
     }
     function invert(point) {
       point = projectRotate.invert((point[0] - δx) / k, (δy - point[1]) / k);
-      return [ point[0] * d3_degrees, point[1] * d3_degrees ];
+      return point && [ point[0] * d3_degrees, point[1] * d3_degrees ];
     }
     projection.stream = function(stream) {
       return d3_geo_projectionRadiansRotate(rotate, clip(projectResample(stream)));

src/geo/compose.js

@@ -5,7 +5,7 @@ function d3_geo_compose(a, b) {
   }
 
   if (a.invert && b.invert) compose.invert = function(x, y) {
-    return x = b.invert(x, y), a.invert(x[0], x[1]);
+    return x = b.invert(x, y), x && a.invert(x[0], x[1]);
   };
 
   return compose;

src/geo/projection.js

@@ -25,7 +25,7 @@ function d3_geo_projectionMutator(projectAt) {
 
   function invert(point) {
     point = projectRotate.invert((point[0] - δx) / k, (δy - point[1]) / k);
-    return [point[0] * d3_degrees, point[1] * d3_degrees];
+    return point && [point[0] * d3_degrees, point[1] * d3_degrees];
   }
 
   projection.stream = function(stream) {

test/geo/projection-test.js

@@ -0,0 +1,21 @@
+require("../env");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.geo.projection");
+
+suite.addBatch({
+  "projection": {
+    topic: function() {
+      function forward(λ, φ) { return [λ, φ]; }
+      forward.invert = function(x, y) {}
+      return d3.geo.projection(forward);
+    },
+    "non-existent inverse": function(projection) {
+      assert.isUndefined(projection.invert([0, 0]));
+    }
+  }
+});
+
+suite.export(module);