Fix d3#127: circular fisheye scaling factor.

This fixes the scaling value `z` when the focus point is exactly on an
object’s position.

Also fixes d3#126.

fisheye/fisheye.js

@@ -14,7 +14,7 @@
         var dx = d.x - focus[0],
             dy = d.y - focus[1],
             dd = Math.sqrt(dx * dx + dy * dy);
-        if (!dd || dd >= radius) return {x: d.x, y: d.y, z: 1};
+        if (!dd || dd >= radius) return {x: d.x, y: d.y, z: dd >= radius ? 1 : 10};
         var k = k0 * (1 - Math.exp(-dd * k1)) / dd * .75 + .25;
         return {x: focus[0] + dx * k, y: focus[1] + dy * k, z: Math.min(k, 10)};
       }