Unfortunately, you cannot bind the tooltip of the axis to some property of the DataPoint. This is why I suggest that you use the TrackBall or the ToolTip behavior of the chart components.
Regards,
Martin Ivanov
Telerik
Check out the new Telerik Platform - the only modular platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native apps. Register for the free online keynote and webinar to learn more about the Platform on Wednesday, February 12, 2014 at 11:00 a.m. ET (8:00 a.m. PT).