DevExpress ChartControl SeriesLabel背景透明

<dxc:SeriesLabel.ElementTemplate>
                                            <DataTemplate>
                                                <Border Background="Transparent" Opacity="0.75" CornerRadius="2" Padding="8,2,8,2">
                                                    <StackPanel>
                                                        <TextBlock Text="{Binding Path=Text}" Foreground="Black"/>
                                                    </StackPanel>
                                                </Border>
                                            </DataTemplate>

</dxc:SeriesLabel.ElementTemplate>