Merge branch 'geom-centroid' of https://github.com/jasondavies/d3 int…

…o centroid

d3.geom.js

@@ -198,7 +198,7 @@ d3.geom.polygon = function(coordinates) {
         a,
         b,
         c;
-    if (!arguments.length) k = 1 / (6 * coordinates.area());
+    if (!arguments.length) k = -1 / (6 * coordinates.area());
     while (++i < n) {
       a = coordinates[i];
       b = coordinates[i + 1];

src/geom/polygon.js

@@ -21,7 +21,7 @@ d3.geom.polygon = function(coordinates) {
         a,
         b,
         c;
-    if (!arguments.length) k = 1 / (6 * coordinates.area());
+    if (!arguments.length) k = -1 / (6 * coordinates.area());
     while (++i < n) {
       a = coordinates[i];
       b = coordinates[i + 1];

test/geom/polygon-test.js

@@ -0,0 +1,57 @@
+require("../env");
+require("../../d3");
+require("../../d3.geom");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.geom.polygon");
+
+suite.addBatch({
+  "counterclockwise polygon (last point equal to start point)": {
+    topic: function() {
+      return d3.geom.polygon([[0, 0], [0, 1], [1, 1], [1, 0], [0, 0]]);
+    },
+    "area": function(polygon) {
+      assert.equal(polygon.area(), 1);
+    },
+    "centroid": function(polygon) {
+      assert.deepEqual(polygon.centroid(), [.5, .5]);
+    }
+  },
+  "counterclockwise polygon (implicitly ending at start point)": {
+    topic: function() {
+      return d3.geom.polygon([[0, 0], [0, 1], [1, 1], [1, 0]]);
+    },
+    "area": function(polygon) {
+      assert.equal(polygon.area(), 1);
+    },
+    "centroid": function(polygon) {
+      assert.deepEqual(polygon.centroid(), [.5, .5]);
+    }
+  },
+  "clockwise polygon (last point equal to start point)": {
+    topic: function() {
+      return d3.geom.polygon([[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]);
+    },
+    "area": function(polygon) {
+      assert.equal(polygon.area(), -1);
+    },
+    "centroid": function(polygon) {
+      assert.deepEqual(polygon.centroid(), [.5, .5]);
+    }
+  },
+  "clockwise polygon (implicitly ending at start point)": {
+    topic: function() {
+      return d3.geom.polygon([[0, 0], [1, 0], [1, 1], [0, 1]]);
+    },
+    "area": function(polygon) {
+      assert.equal(polygon.area(), -1);
+    },
+    "centroid": function(polygon) {
+      assert.deepEqual(polygon.centroid(), [.5, .5]);
+    }
+  }
+});
+
+suite.export(module);