Toggling RangeSlider so xMin = xMax causes javascript errors

When viewing a graph with a RangeSlider, it's possible to move the sliders such that xMin = xMax (i.e. overlapping min/max buttons on the slider). This causes some javascript errors.

In particular, this code in Rickshaw.Graph:

```javascript
		var data = this.series.active()
			.map( function(d) { return d.data } )
			.map( function(d) { return d.filter( function(d) { return this._slice(d) }, this ) }, this); 
```

results in data[0].length === 0, which causes this code in Rickshaw.Graph.Renderer.Bar:

```javascript
			var xMin = stackedData[0][0].x;
			var xMax = stackedData[0][ stackedData[0].length - 1 ].x;
```

to blow up since stackedData[0][0].x is undefined. 

The commit ensures that stackedData[0][0] always exists. However, I have a feeling there's a better solution out there... while these commits seem to fix the javascript errors, Chrome 16 still reports this error in the console: Error: Invalid value for rect attribute height="NaN"