如何“堆叠”Silverlight条形图?

假设我有这个XAML(用 Live ChartBuilder ):

<Grid xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:sys="clr-namespace:System;assembly=mscorlib" xmlns:datavis="clr-namespace:System.Windows.Controls.DataVisualization;assembly=System.Windows.Controls.DataVisualization.Toolkit" xmlns:charting="clr-namespace:System.Windows.Controls.DataVisualization.Charting;assembly=System.Windows.Controls.DataVisualization.Toolkit" xmlns:utility="clr-namespace:Utility;assembly=ChartBuilder" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
  <charting:Chart>
    <charting:Chart.Series>
      <charting:BarSeries
        Title="Series 1">
        <charting:BarSeries.ItemsSource>
          <utility:ObservableObjectCollection>
            <sys:Double>1</sys:Double>
            <sys:Double>2.33333325386047</sys:Double>
            <sys:Double>3.66666674613953</sys:Double>
            <sys:Double>5</sys:Double>
          </utility:ObservableObjectCollection>
        </charting:BarSeries.ItemsSource>
      </charting:BarSeries>
      <charting:BarSeries
        Title="Series 2">
        <charting:BarSeries.ItemsSource>
          <utility:ObservableObjectCollection>
            <sys:Double>1</sys:Double>
            <sys:Double>2.33333325386047</sys:Double>
            <sys:Double>3.66666674613953</sys:Double>
            <sys:Double>5</sys:Double>
          </utility:ObservableObjectCollection>
        </charting:BarSeries.ItemsSource>
      </charting:BarSeries>
    </charting:Chart.Series>
  </charting:Chart>
</Grid>

它生成一个带有两个水平显示条的条形图。我怎样才能把它们叠起来?