diff -r efd9c589177a -r c0b4a8b5a012 toolkit/javascript/d3/src/geo/circle.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/javascript/d3/src/geo/circle.js	Thu Apr 10 14:20:23 2014 +0200
@@ -0,0 +1,146 @@
+// TODO breakAtDateLine?
+
+d3.geo.circle = function() {
+  var origin = [0, 0],
+      degrees = 90 - 1e-2,
+      radians = degrees * d3_geo_radians,
+      arc = d3.geo.greatArc().target(Object);
+
+  function circle() {
+    // TODO render a circle as a Polygon
+  }
+
+  function visible(point) {
+    return arc.distance(point) < radians;
+  }
+
+  circle.clip = function(d) {
+    arc.source(typeof origin === "function" ? origin.apply(this, arguments) : origin);
+    return clipType(d);
+  };
+
+  var clipType = d3_geo_type({
+
+    FeatureCollection: function(o) {
+      var features = o.features.map(clipType).filter(Object);
+      return features && (o = Object.create(o), o.features = features, o);
+    },
+
+    Feature: function(o) {
+      var geometry = clipType(o.geometry);
+      return geometry && (o = Object.create(o), o.geometry = geometry, o);
+    },
+
+    Point: function(o) {
+      return visible(o.coordinates) && o;
+    },
+
+    MultiPoint: function(o) {
+      var coordinates = o.coordinates.filter(visible);
+      return coordinates.length && {
+        type: o.type,
+        coordinates: coordinates
+      };
+    },
+
+    LineString: function(o) {
+      var coordinates = clip(o.coordinates);
+      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);
+    },
+
+    MultiLineString: function(o) {
+      var coordinates = o.coordinates.map(clip).filter(function(d) { return d.length; });
+      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);
+    },
+
+    Polygon: function(o) {
+      var coordinates = o.coordinates.map(clip);
+      return coordinates[0].length && (o = Object.create(o), o.coordinates = coordinates, o);
+    },
+
+    MultiPolygon: function(o) {
+      var coordinates = o.coordinates.map(function(d) { return d.map(clip); }).filter(function(d) { return d[0].length; });
+      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);
+    },
+
+    GeometryCollection: function(o) {
+      var geometries = o.geometries.map(clipType).filter(Object);
+      return geometries.length && (o = Object.create(o), o.geometries = geometries, o);
+    }
+
+  });
+
+  function clip(coordinates) {
+    var i = -1,
+        n = coordinates.length,
+        clipped = [],
+        p0,
+        p1,
+        p2,
+        d0,
+        d1;
+
+    while (++i < n) {
+      d1 = arc.distance(p2 = coordinates[i]);
+      if (d1 < radians) {
+        if (p1) clipped.push(d3_geo_greatArcInterpolate(p1, p2)((d0 - radians) / (d0 - d1)));
+        clipped.push(p2);
+        p0 = p1 = null;
+      } else {
+        p1 = p2;
+        if (!p0 && clipped.length) {
+          clipped.push(d3_geo_greatArcInterpolate(clipped[clipped.length - 1], p1)((radians - d0) / (d1 - d0)));
+          p0 = p1;
+        }
+      }
+      d0 = d1;
+    }
+
+    if (p1 && clipped.length) {
+      d1 = arc.distance(p2 = clipped[0]);
+      clipped.push(d3_geo_greatArcInterpolate(p1, p2)((d0 - radians) / (d0 - d1)));
+    }
+
+    return resample(clipped);
+  }
+
+  // Resample coordinates, creating great arcs between each.
+  function resample(coordinates) {
+    var i = 0,
+        n = coordinates.length,
+        j,
+        m,
+        resampled = n ? [coordinates[0]] : coordinates,
+        resamples,
+        origin = arc.source();
+
+    while (++i < n) {
+      resamples = arc.source(coordinates[i - 1])(coordinates[i]).coordinates;
+      for (j = 0, m = resamples.length; ++j < m;) resampled.push(resamples[j]);
+    }
+
+    arc.source(origin);
+    return resampled;
+  }
+
+  circle.origin = function(x) {
+    if (!arguments.length) return origin;
+    origin = x;
+    return circle;
+  };
+
+  circle.angle = function(x) {
+    if (!arguments.length) return degrees;
+    radians = (degrees = +x) * d3_geo_radians;
+    return circle;
+  };
+
+  // Precision is specified in degrees.
+  circle.precision = function(x) {
+    if (!arguments.length) return arc.precision();
+    arc.precision(x);
+    return circle;
+  };
+
+  return circle;
+}