Fallback subject.

Reverts most of 8b25a53. Fixes #18.

README.md

@@ -122,13 +122,13 @@ If *subject* is specified, sets the subject accessor to the specified object or
 
 ```js
 function subject(d) {
-  return d;
+  return d == null ? this : d;
 }
 ```
 
 The *subject* of a drag gesture represents *the thing being dragged*. It is computed when an initiating input event is received, such as a mousedown or touchstart, immediately before the drag gesture starts. The subject is then exposed as *event*.subject on subsequent [drag events](#drag-events) for this gesture.
 
-The default subject is the [datum](https://github.com/d3/d3-selection#selection_datum) of the element in the originating selection (see [*drag*](#_drag)) that received the initiating input event. When dragging circle elements in SVG, the default subject is the datum of the circle being dragged; with [Canvas](https://html.spec.whatwg.org/multipage/scripting.html#the-canvas-element), the default subject is the canvas element’s datum (regardless of where on the canvas you click). In this case, a custom subject accessor would be more appropriate, such as one that picks the closest circle to the mouse within a given search *radius*:
+The default subject is the [datum](https://github.com/d3/d3-selection#selection_datum) of the element in the originating selection (see [*drag*](#_drag)) that received the initiating input event; if the datum is undefined, the default subject is the element itself. When dragging circle elements in SVG, the default subject is the datum of the circle being dragged. With [Canvas](https://html.spec.whatwg.org/multipage/scripting.html#the-canvas-element), the default subject is the canvas element’s datum (regardless of where on the canvas you click). In this case, a custom subject accessor would be more appropriate, such as one that picks the closest circle to the mouse within a given search *radius*:
 
 ```js
 function subject() {

src/drag.js

@@ -15,7 +15,7 @@ function defaultContainer() {
 }
 
 function defaultSubject(d) {
-  return d;
+  return d == null ? this : d;
 }
 
 function defaultX() {