XAML:

    xmlns:tk="clr-namespace:System.Windows.Controls.DataVisualization.Charting;assembly=System.Windows.Controls.DataVisualization.Toolkit" 
    Title="One Line" Height="350" Width="525" Loaded="Window_Loaded">
    <DockPanel LastChildFill="True">
        <tk:Chart Canvas.Top="80" Canvas.Left="10" Name="mcChart"  Background="LightSteelBlue">
            <tk:Chart.Series>
                <tk:LineSeries  BorderThickness="1" Title=" Monthly Count" IndependentValueBinding="{Binding Path=Key}" DependentValueBinding="{Binding Path=Value}" />
                <tk:LineSeries Title=" Monthly Count2" IndependentValueBinding="{Binding Path=Key}" DependentValueBinding="{Binding Path=Value}" />
            </tk:Chart.Series>
        </tk:Chart>
    </DockPanel>


CS:
using System.Windows.Controls.DataVisualization.Charting;

private void Window_Loaded(object sender, RoutedEventArgs e) {
    mcChart.LegendTitle = "Legend";
   ((LineSeries)mcChart.Series[0]).ItemsSource =
      new KeyValuePair<DateTime, int>[]{
      new KeyValuePair<DateTime,int>(DateTime.Now, 100),
      new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(1), 130),
      new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(2), 150),
      new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(3), 125),
      new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(4),155) };

  LineSeries ls2 = ((LineSeries)mcChart.Series[1]);
            ls2.ItemsSource =
        new KeyValuePair<DateTime, int>[]{
        new KeyValuePair<DateTime,int>(DateTime.Now, 200),
        new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(3), 230),
        new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(5), 250),
        new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(6), 225),
        new KeyValuePair<DateTime,int>(DateTime.Now.AddMonths(7),255) };
}