Add selection.clone.

Fixes #149.

Author: mbostock

README.md

@@ -386,6 +386,16 @@ The specified *name* may have a namespace prefix, such as `svg:text` to specify
 
 Removes the selected elements from the document. Returns this selection (the removed elements) which are now detached from the DOM. There is not currently a dedicated API to add removed elements back to the document; however, you can pass a function to [*selection*.append](#selection_append) or [*selection*.insert](#selection_insert) to re-add elements.
 
+<a name="selection_clone" href="#selection_clone">#</a> <i>selection</i>.<b>clone</b>([<i>deep</i>]) [<>](https://github.com/d3/d3-selection/blob/master/src/selection/clone.js "Source")
+
+Inserts clones the selected elements immediately following the selected elements. Equivalent to:
+
+```js
+selection.select(function() {
+  return this.parentNode.insertBefore(this.cloneNode(deep), this.nextSibling);
+});
+```
+
 <a name="selection_sort" href="#selection_sort">#</a> <i>selection</i>.<b>sort</b>(<i>compare</i>) [<>](https://github.com/d3/d3-selection/blob/master/src/selection/sort.js "Source")
 
 Returns a new selection that contains a copy of each group in this selection sorted according to the *compare* function. After sorting, re-inserts elements to match the resulting order (per [*selection*.order](#selection_order)).

src/selection/clone.js

@@ -0,0 +1,11 @@
+function selection_cloneShallow() {
+  return this.parentNode.insertBefore(this.cloneNode(false), this.nextSibling);
+}
+
+function selection_cloneDeep() {
+  return this.parentNode.insertBefore(this.cloneNode(true), this.nextSibling);
+}
+
+export default function(deep) {
+  return this.select(deep ? selection_cloneDeep : selection_cloneShallow);
+}

src/selection/index.js

@@ -24,6 +24,7 @@ import selection_lower from "./lower";
 import selection_append from "./append";
 import selection_insert from "./insert";
 import selection_remove from "./remove";
+import selection_clone from "./clone";
 import selection_datum from "./datum";
 import selection_on from "./on";
 import selection_dispatch from "./dispatch";
@@ -67,6 +68,7 @@ Selection.prototype = selection.prototype = {
   append: selection_append,
   insert: selection_insert,
   remove: selection_remove,
+  clone: selection_clone,
   datum: selection_datum,
   on: selection_on,
   dispatch: selection_dispatch