keep a copy of starting positions

`label_array[i].sx` and `label_array[i].sy` are a copy of the initial positions; makes it easy to use the distance moved to change the label or the label class

Basic example:

```
.text(function(d) { return d.name + ' ' + Math.ceil(Math.sqrt((d.x-d.sx)*(d.x-d.sx) + (d.y-d.sy)*(d.y-d.sy))) });
```