如何在Telerik的RadCartesianChart的工具提示中显示不受CategoryPath或ValuePath约束的值？

<telerik:RadCartesianChart>
    <telerik:RadCartesianChart.HorizontalAxis>
        <telerik:CategoricalAxis />
    </telerik:RadCartesianChart.HorizontalAxis>
    <telerik:RadCartesianChart.VerticalAxis>
        <telerik:LinearAxis />
    </telerik:RadCartesianChart.VerticalAxis>
    <telerik:RadCartesianChart.Series>
        <telerik:BarSeries ItemsSource="{Binding Data}">
            <telerik:BarSeries.ToolTipTemplate>
                <DataTemplate>
                    <StackPanel>
                        <TextBlock Text="{Binding Category}" />
                        <TextBlock Text="{Binding Value}" />
                        <TextBlock Text="{Binding UnconstrainedValue}" />
                    </StackPanel>
                </DataTemplate>
            </telerik:BarSeries.ToolTipTemplate>
        </telerik:BarSeries>
    </telerik:RadCartesianChart.Series>
</telerik:RadCartesianChart>

public class ViewModel
{
    public ObservableCollection<DataPoint> Data { get; set; }

    public ViewModel()
    {
        Data = new ObservableCollection<DataPoint>
        {
            new DataPoint { Category = "Category 1", Value = 10, UnconstrainedValue = 15 },
            new DataPoint { Category = "Category 2", Value = 20, UnconstrainedValue = 25 },
            new DataPoint { Category = "Category 3", Value = 30, UnconstrainedValue = 35 }
        };
    }
}

public class DataPoint
{
    public string Category { get; set; }
    public double Value { get; set; }
    public double UnconstrainedValue { get; set; }
}