折线的绑定.我做错了什么?

我正在尝试我认为一个简单的事情:画线的点列表。如果我把列表静态地放在窗口的xaml中，一切都没问题。如果执行绑定，则不会显示任何内容。

窗口代码:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>
    <Polyline Stretch="Fill" Grid.Column="0" Name="polyline" Stroke="Red" DataContext="{Binding Points}">
    </Polyline>   
</Grid>

public partial class testWindow2 : Window
{
    AudioSignalModelView audioSignalModelView;
    public testWindow2()
    {
        InitializeComponent();
        audioSignalModelView = new AudioSignalModelView();
        this.DataContext = audioSignalModelView;
    }
}
public class AudioSignalModelView
{
    public AudioSignalModelView()
    {
        Point pointA = new Point {X=0,Y=0};
        Point pointB = new Point { X = 0.2, Y = 0.4 };
        Point pointC = new Point { X = 0.8, Y = 0.1 };
        Point pointD = new Point { X = 1, Y = 1 };
        Points.Add(pointA);
        Points.Add(pointB);
        Points.Add(pointC);
        Points.Add(pointD);
    }
    private AudioSignalTest audioSignalTest;
    private PointCollection _points = new PointCollection();
    public PointCollection Points
    {
        get { return _points; }
    }
}

我认为绑定是以某种方式完成的，因为如果我在Points属性的getter中放置了一个断点，它将被系统调用…

我的代码中有什么明显的错误?

<Polyline Stretch="Fill" Grid.Column="0"
          Name="polyline" Stroke="Red"
          Points="{Binding Points}">  <-- Here
</Polyline>