Tooltip doesn't display the same way if you add or remove XAxis.

Without XAxis the tooltip doesn't know the datakey and so display a number.
http://jsfiddle.net/areski/97q4apgw/2/

With XAxis we got a perfect tooltip:
http://jsfiddle.net/areski/97q4apgw/5/