WPF Charting Toolkit: How to set a fixed X-axis range while adding real time data

I'm doing an application real time data charting basing on WPF Charting Toolkit. I get the datas via serial port. The code of setting chart is below:

    <chartingToolkit:Chart  Margin="10,10,10,0" ClipToBounds="True" x:Name="chart1" Title="Chart Title">
        <chartingToolkit:LineSeries IndependentValueBinding="{Binding Value1}" DependentValueBinding="{Binding Value2}" ItemsSource="{Binding}" Background="Transparent" Cursor="No">
            <chartingToolkit:LineSeries.DataPointStyle>
                <Style TargetType="{x:Type chartingToolkit:LineDataPoint}">
                    <Setter Property="Height" Value="0"/>
                    <Setter Property="Width" Value="0" />
                    <Setter Property="Background" Value="Green"/>
                </Style>
            </chartingToolkit:LineSeries.DataPointStyle>
        </chartingToolkit:LineSeries>
    </chartingToolkit:Chart>

It works good but I still need to set maximum and minimum values of X axis. The X Values (Value1) are as number of received samples and the Y axis values (Value2) are obviously as concrete values of the received samples.

My question is about the X axis range.

Currently, I'm getting minimum as 0 and maximinum as the highest number of sample which the serial port received in current moment.

But I want to set a permanent range of X axis which I want to see.

For example I want to see on the X axis range of 500 samples.

It means that when the number of samples exceeds 500, the max should be as the highest sample number and the min should be max-500.

The main difficulty is how to set it with real time data in WPF??

Can anyone help me, please??

Updated question

I'm updating my question after @jstreet advise.

I have this method which is running in separate thread within MainWindow class, likes below.

 public partial class MainWindow : Window 
  {
 public SerialPort serialPort1 = new SerialPort();
    public string rx_str = "";
    public string rx_str_copy;
    public int a;
    public double x, y;


      ObservableCollection<ChartData> chartData;
    ChartData objChartData;
    Thread myThread;

     public MainWindow()
    {
        InitializeComponent();
        string[] port = SerialPort.GetPortNames();
        foreach (string a in port)
        {
            comboPorts.Items.Add(a);
        }
        Array.Sort(port);
        comboPorts.Text = port[0];

        objChartData = new ChartData();
        chartData.Add(objChartData);
        chart1.DataContext = chartData;
        myThread = new Thread(new ThreadStart(Run));


    }

   public void Run()
       {
        while (true)
        {
            serialPort1.Write("a");
            rx_str = serialPort1.ReadTo("b");
            rx_str_copy = rx_str;
            x = a;
            y = Double.Parse(rx_str_copy,     CultureInfo.InvariantCulture);                                               

            a++;

            Dispatcher.Invoke(new Action(delegate
            {

         chartData.Add(new ChartData() { Value1 = x,         
       Value2= y             });          
            }));



        }
    }  

This Run() method is responsible for receiving datas and adding it to the chart.

In another class I have handle of reaction on comming datas and settings properties Valeu1 and Value2:

  public class ChartData : INotifyPropertyChanged
   {
    double _Value1;
    double _Value2;


    public double Value1
    {
        get
        {
            return _Value1;
        }
        set
        {
            _Value1 = value;
            OnPropertyChanged("Value1");
        }
    }

    public double Value2
    {
        get
        {
            return _Value2;
        }
        set
        {
            _Value2 = value;
            OnPropertyChanged("Value2");
        }
    }


    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new   
     PropertyChangedEventArgs(propertyName));
        }
    }
}

How can I adapt @jstreet's solution to my behind code example??