Add transpose.

Makefile

@@ -53,6 +53,7 @@ d3.core.js: \
 	src/core/number.js \
 	src/core/sum.js \
 	src/core/quantile.js \
+	src/core/transpose.js \
 	src/core/zip.js \
 	src/core/bisect.js \
 	src/core/first.js \

d3.js

@@ -175,6 +175,9 @@ d3.quantile = function(values, p) {
       e = H - h;
   return e ? v + e * (values[h] - v) : v;
 };
+d3.transpose = function(matrix) {
+  return d3.zip.apply(d3, matrix);
+};
 d3.zip = function() {
   if (!(n = arguments.length)) return [];
   for (var i = -1, m = d3.min(arguments, d3_zipLength), zips = new Array(m); ++i < m;) {

src/core/transpose.js

@@ -0,0 +1,3 @@
+d3.transpose = function(matrix) {
+  return d3.zip.apply(d3, matrix);
+};

test/core/transpose-test.js

@@ -0,0 +1,32 @@
+require("../env");
+require("../../d3");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.transpose");
+
+suite.addBatch({
+  "transpose": {
+    topic: function() {
+      return d3.transpose;
+    },
+    "transposes a square matrix": function(transpose) {
+      assert.deepEqual(d3.transpose([[1, 2], [3, 4]]), [[1, 3], [2, 4]]);
+    },
+    "transposes a non-square matrix": function(transpose) {
+      assert.deepEqual(d3.transpose([[1, 2, 3, 4, 5], [2, 4, 6, 8, 10]]), [[1, 2], [2, 4], [3, 6], [4, 8], [5, 10]]);
+    },
+    "transposes a single-row matrix": function(transpose) {
+      assert.deepEqual(d3.transpose([[1, 2, 3, 4, 5]]), [[1], [2], [3], [4], [5]]);
+    },
+    "transposes an empty matrix": function(transpose) {
+      assert.deepEqual(d3.transpose([]), []);
+    },
+    "ignores extra elements given an irregular matrix": function(transpose) {
+      assert.deepEqual(d3.transpose([[1, 2], [3, 4], [5, 6, 7]]), [[1, 3, 5], [2, 4, 6]]);
+    }
+  }
+});
+
+suite.export(module);