Hello again!

With the help of this thread http://ko.infragistics.com/community/forums/t/80062.aspx I found the final solution.

I added a style to the XAML file where the grouped data is shown and modified the Tooltip property.

<Style TargetType="{x:Type igWPF:GroupByRecordPresenter}">
  <Setter Property="ToolTip" >
    <Setter.Value>
      <StackPanel Orientation="Vertical">
        <Label Content="{Binding Converter={StaticResource GroupToolTipConv}}" />
        <Image Source="{Binding Converter={StaticResource GroupToolTipImageConverter}}" />
      </StackPanel>
    </Setter.Value>
  </Setter>
</Style>

If someone still needs the MouseEnter Event on the group header just add the following to your the GroupByRecordPresenter Style

<EventSetter Event="MouseEnter" Handler="DataRecordPresenterGroup_MouseEnter" />

And in the code behind file

void DataRecordPresenterGroup_MouseEnter(object sender, MouseEventArgs e)

{

   GroupByRecordPresenter groupPresenter = sender as GroupByRecordPresenter;

   if (groupPresenter.DataContext != null)

      {

     GroupByRecord rec = groupPresenter.DataContext as GroupByRecord;

     if (rec.HasChildren)

          {

       if (rec.ChildRecords.Count > 0)

              {

         var x = ((rec.ChildRecords[0] as DataRecord).DataItem) as ChartData;

              }

          }

     }

}

Attached please find the complete solution.