Updating UserControl (Chart) in MVVM

Updating UserControl (Chart) in MVVM

I have a chart embedded in a usercontrol
PlotControl.xaml (PlotControl.xaml.cs)
This plot control is used on a View in MVVM.
NOW! when I update the UI of PlotControl (e.g. I draw vertical and
horizontal markers on chart), these updates are not visible on View
(unless I do double click on the View or do a Minimize-Maximize window).
Is there a way the updated UI is updated automatically on View?
View code looks like:
<Grid Margin="4">
<nms:PlotControl x:Name="PlotControl" Margin="10,10"
DockPanel.Dock="Right" />
</nms:PlotControl>
Snippet from PlotControl.xaml.cs code looks like:
ChartPanel cpnl = new ChartPanel();
chart.View.Layers.Add(cpnl);
ChartMarker rightVerticalMarker = new ChartMarker(chart,
MarketType.RightVertical);
rightVerticalMarker.DataPoint = new Point(30, double.NaN);
cpnl.Children.Add(rightVerticalMarker);
cpnl.UpdateLayout();
chart.UpdateLayout();
ChartMarker is simply a line Horizontal (or Vertical) defined by enum
MarketType.