Silverlight - Width of RadPanelBar in StackPanel

I have two controls in a StackPanel; one is a RadChart and the other one is RadPanelBar. It shows 100% width for RadChart but it is not showing 100% width for the RadPanelBar.

I am using the following code. Can someone tell what 开发者_如何学编程could be the issue, why it does not show 100% width for RadPanelBar

 <StackPanel x:Name="ContentStackPanel" Style="{StaticResource ContentStackPanelStyle}">
                    <telerikChart:RadChart x:Name="radChart" Margin="8">
                        <telerikChart:RadChart.SeriesMappings>
                            <telerik:SeriesMapping LegendLabel="Results">
                                <telerik:SeriesMapping.ItemMappings>
                                    <telerik:ItemMapping FieldName="Score" DataPointMember="YValue"/>
                                </telerik:SeriesMapping.ItemMappings>
                            </telerik:SeriesMapping>
                        </telerikChart:RadChart.SeriesMappings>
                    </telerikChart:RadChart>-->

                    <telerik:RadPanelBar x:Name="radPanelBar"
                              HorizontalAlignment="Center" VerticalAlignment="Top"
                              ItemTemplate="{StaticResource PanelBarHeaderTemplate}">
                    </telerik:RadPanelBar>

            </StackPanel>

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Change from HorizontalAlignment and VerticalAlignment to Stretch might work; please try.