Bug Fix - Prevents tooltip from showing with Date data type in line charts

Tooltips do not work when using Date objects for X or Y values with line charts.  The root of the problem comes from trying to add a "jitter" to these points.  In firebug I receive the following warning:

`Unexpected value NaN parsing cx attribute.`

My fix checks for a Date type first before applying the jitter which removes the NaN warning and fixes the tooltip.

(Sorry for the large chunk of changes in this commit I couldn't figure out why git read the change this way.  I only made changes to lines 10,031 - 10,035 )