Add d3.geoStereographic.

index.js

@@ -12,6 +12,7 @@ export {default as geoEquirectangular} from "./src/projection/equirectangular";
 export {default as geoProjection, projectionMutator as geoProjectionMutator} from "./src/projection/index";
 export {default as geoMercator} from "./src/projection/mercator";
 export {default as geoOrthographic} from "./src/projection/orthographic";
+export {default as geoStereographic} from "./src/projection/stereographic";
 export {default as geoTransverseMercator} from "./src/projection/transverseMercator";
 export {default as geoRotation} from "./src/rotation";
 export {default as geoStream} from "./src/stream";

src/projection/stereographic.js

@@ -0,0 +1,16 @@
+import {atan, cos, sin} from "../math";
+import {azimuthalInvert} from "./azimuthal";
+import projection from "./index";
+
+function stereographic(x, y) {
+  var cy = cos(y), k = 1 + cos(x) * cy;
+  return [cy * sin(x) / k, sin(y) / k];
+}
+
+stereographic.invert = azimuthalInvert(function(z) {
+  return 2 + atan(z);
+});
+
+export default function() {
+  return projection(stereographic);
+}