Chart not drawing correctly

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C#

Windows-Store

I trying to create line chart using WinRT XAML Toolkit[^] which is a port of SilverlightToolkit. The problem is values are displayed but I have no line(nothing is draw) in my LineChart. I followed the method used in this site[^]

The result I get is http://i.imgur.com/IK3suOZ.jpg[^]

Xaml I used for drawing chart is:

<Charting:Chart x:Name="lineChart" HorizontalAlignment="Left" Height="460" Margin="422,53,0,0" Grid.Row="1" VerticalAlignment="Top" Width="793" Loaded="Chart_Loaded">
    <Charting:LineSeries Height="100" Width="100" IndependentValuePath="ValueCode" DependentValuePath="ExchangeRate" IsSelectionEnabled="True"/>
</Charting:Chart>

And C# code behind:

private void Chart_Loaded(object sender, RoutedEventArgs e)
{
    List<testClass> test2 = new List<testClass>();
    test2.Add(new testClass() { ValueCode = "USD", ExchangeRate =2});
    test2.Add(new testClass() { ValueCode = "USD", ExchangeRate = 4 });
    test2.Add(new testClass() { ValueCode = "USD", ExchangeRate = 6 });

    (lineChart.Series[0] as LineSeries).ItemsSource = test2;
}

testClass code:

public class testClass
{

    string value_code;
    int exchange_rate;
       
    public int ExchangeRate
    {
        get { return exchange_rate; }
        set { exchange_rate = value; }
    }

    public string ValueCode
    {
        get { return value_code; }
        set { value_code = value; }
    }
}

I've tested LineChart, ColumnChart and PieChart - PieChart is the only one working as there are no lines,columns to draw.

(I am using latest version of WinRT XAML Toolkit)