title: DataGrid ItemsSource Cell value changed
body: <p>I have a custom data grid, the datagrid display a collection of coordiantes: </p>
  
  <pre><code>&lt;customControls:CustomDataGrid ItemsSource="{Binding Points}"              
                SelectedItem="{Binding SelectedRow}" IsSynchronizedWithCurrentItem="True"      
                CanUserAddRows="True" SelectionMode="Extended" AutoGenerateColumns="False"&gt; 
      &lt;DataGrid.Columns&gt;                                                                         
        &lt;DataGridTextColumn  Header="X" Binding="{Binding X}" Width="Auto"/&gt;
        &lt;DataGridTextColumn Header="Y" Binding="{Binding Y}" Width="Auto"/&gt;
      &lt;/DataGrid.Columns&gt;   
  &lt;/customControls:CustomDataGrid &gt;
  </code></pre>
  
  <p>I need a way to get notified when a cell value has changed (not a row change).</p>