Add d3.zip.

Analagous to Python's built-in zip():
<http://docs.python.org/library/functions.html#zip>

Fixes #145.

Makefile

@@ -37,6 +37,7 @@ d3.core.js: \
 	src/core/descending.js \
 	src/core/min.js \
 	src/core/max.js \
+	src/core/zip.js \
 	src/core/bisect.js \
 	src/core/nest.js \
 	src/core/keys.js \
@@ -167,6 +168,7 @@ tests: \
 	tests/test-keys.test \
 	tests/test-nest.test \
 	tests/test-permute.test \
+	tests/test-zip.test \
 	tests/test-remove.test \
 	tests/test-rgb.test \
 	tests/test-round.test \

d3.js

@@ -66,6 +66,16 @@ d3.max = function(array, f) {
   }
   return a;
 };
+d3.zip = function() {
+  var n = arguments.length,
+      length = d3.max(arguments, function(d) { return d.length; }),
+      results = new Array(length);
+  for (var i = 0; i < length; i++) {
+    results[i] = new Array(n);
+    for (var j = 0; j < length; j++) results[i][j] = arguments[j][i];
+  }
+  return results;
+};
 // Locate the insertion point for x in a to maintain sorted order. The
 // arguments lo and hi may be used to specify a subset of the array which should
 // be considered; by default the entire array is used. If x is already present

src/core/zip.js

@@ -0,0 +1,10 @@
+d3.zip = function() {
+  var n = arguments.length,
+      length = d3.max(arguments, function(d) { return d.length; }),
+      results = new Array(length);
+  for (var i = 0; i < length; i++) {
+    results[i] = new Array(n);
+    for (var j = 0; j < length; j++) results[i][j] = arguments[j][i];
+  }
+  return results;
+};

tests/test-zip.js

@@ -0,0 +1,10 @@
+require("./../lib/env-js/envjs/node");
+require("./../d3");
+
+console.log("zip [1, 2] [3, 4]:");
+console.log("  " + d3.zip([1, 2], [3, 4]));
+console.log("");
+
+console.log("zip [1, 2] [3, 4] [5, 6, 7]:");
+console.log("  " + d3.zip([1, 2], [3, 4], [5, 6, 7]));
+console.log("");

tests/test-zip.out

@@ -0,0 +1,6 @@
+zip [1, 2] [3, 4]:
+  1,3,2,4
+
+zip [1, 2] [3, 4] [5, 6, 7]:
+  1,3,5,2,4,6,,,7
+