Hi, 
The status of this bug is fixed, but i seems to have a similiar problem with a different type of chart - when i use the grouping based on this http://www.telerik.com/support/kb/aspnet-ajax/chart-(html5)/details/displaying-grand-total-in-stacked-series-labels-and-pie-series-legend
when a column is null, hover over the column (or over the null word) will throw a js error:
Telerik.Web.UI.WebResource.axd:38606 Uncaught TypeError: Cannot read property 'toString' of null
    at init._pointContent (Telerik.Web.UI.WebResource.axd:38606)
    at init.show (Telerik.Web.UI.WebResource.axd:38579)
    at init._showTooltip (Telerik.Web.UI.WebResource.axd:38413)
    at aX.trigger (Telerik.Web.UI.WebResource.axd:29535)
    at init.trigger (Telerik.Web.UI.WebResource.axd:37827)
    at init.notify (Telerik.Web.UI.WebResource.axd:30349)
    at aX.show (Telerik.Web.UI.WebResource.axd:34792)
    at aX.show (Telerik.Web.UI.WebResource.axd:36534)
    at init._startHover (Telerik.Web.UI.WebResource.axd:37986)
    at init._mouseover (Telerik.Web.UI.WebResource.axd:37989)
_pointContent @ Telerik.Web.UI.WebResource.axd:38606
show @ Telerik.Web.UI.WebResource.axd:38579
_showTooltip @ Telerik.Web.UI.WebResource.axd:38413
trigger @ Telerik.Web.UI.WebResource.axd:29535
trigger @ Telerik.Web.UI.WebResource.axd:37827
notify @ Telerik.Web.UI.WebResource.axd:30349
show @ Telerik.Web.UI.WebResource.axd:34792
show @ Telerik.Web.UI.WebResource.axd:36534
_startHover @ Telerik.Web.UI.WebResource.axd:37986
_mouseover @ Telerik.Web.UI.WebResource.axd:37989
trigger @ Telerik.Web.UI.WebResource.axd:20624
_triggerInstanceEvent @ Telerik.Web.UI.WebResource.axd:29292
e @ Telerik.Web.UI.WebResource.axd:1571
trigger @ Telerik.Web.UI.WebResource.axd:20624
(anonymous) @ Telerik.Web.UI.WebResource.axd:27038

is this the same type of error as in this tracked item or not?
if there is no fix for it yet, is there a workaround for this?

thanks,
ilan.