Currently, this is not possible. You need to change the value of the data source.

So...  A two-way binding defined within the XAML is not possible?  Only code-behind/attached properties will allow me to update the active record in my xamdatagrid.  True/False?

Thx

You can use:

TestDataItem dataItem = this.selectedDataPoint.DataContext as TestDataItem;

Or this...

if ( selectedDataPoint == null )
	return;
var index = ( (Series) selectedDataPoint.Parent ).DataPoints.IndexOf(selectedDataPoint);
var dataItem = ( (IEnumerable<TestDataItem>) ( (Series) selectedDataPoint.Parent ).DataSource ).ElementAt(index);

I added this code to my mouse button up event handler.  It works, but it is kind of smelly to me.  Two-Way binding would be much clearner. 

var fieldInfo = typeof (DataPoint).GetField("_oldBindingObject", BindingFlags.NonPublic | BindingFlags.Instance);
if ( fieldInfo != null )
{
 
	var dataItem = (TestDataItem) fieldInfo.GetValue(selectedDataPoint);
	dataItem.Value = selectedDataPoint.Value;
	selectedDataPoint.ToolTip = dataItem.Tooltip;
}