title: change point size in WPFToolkit
body: <p>i try to change point size in wpf toolkit </p>
  
  <pre><code>&lt;charting:Chart Name="Charts" Margin="87,48,0,0" Grid.Column="1"&gt;
          &lt;charting:LineSeries Name="ChartOne" DependentValuePath="Y" IndependentValuePath="X" Title="График функции" AnimationSequence="FirstToLast"/&gt;
          &lt;charting:Chart.Axes&gt;
              &lt;charting:LinearAxis Orientation="Y" Title="Y" ShowGridLines="True" Minimum="0"/&gt;
              &lt;charting:LinearAxis Orientation="X" Title="X" ShowGridLines="True" Minimum="0"/&gt;
          &lt;/charting:Chart.Axes&gt;
          &lt;charting:LineSeries.DataPointStyle&gt;
              &lt;Style TargetType="chartingToolkit:LineDataPoint"&gt;
                  &lt;Setter Property="Width" Value="20"/&gt;
                  &lt;Setter Property="Height" Value="20"/&gt;
              &lt;/Style&gt;
          &lt;/charting:LineSeries.DataPointStyle&gt;
      &lt;/charting:Chart&gt;
  </code></pre>
  
  <p>but i have an error "The attachable property 'DataPointStyle' was not found in type 'LineSeries'"
  what am I doing wrong ?</p>