data binding other than index

D3.md

@@ -111,6 +111,15 @@ d3.selectAll('circle')//make a "ghost call" to all circles, even if there are no
 	.remove(); //removes those elements
 ```
 
+To bind data to elements by something other than index:
+
+```javascript
+.data(data, function(d){
+		//match data based on d.id, not index
+		return d.id
+});
+```
+
 ## Linear Scale
 
 A scale will map a data value to a visual value.