I'm adding multiple ValueOverlays to the xamDataChart via Code Behind based on the DataContext (VM). If I zoom in to the ValueOverlays they seem to plot perfectly. When I am zoomed completely out the Overlays overlap because Thickness is not scaling correctly. The code snippet below shows what I'm trying to accomplish; I am trying to plot a Depth Zone over a Scatter Line Series. I use the average as the Value Property (middle) and the difference as the Thickness Property.

ValueOverlay overlay = new ValueOverlay();

Binding binding = new Binding("Description");
binding.Source = zone;
binding.Converter = new ZoneTitleConverter();
BindingOperations.SetBinding(overlay, Series.TitleProperty, binding);                   
overlay.Axis = LineYAxis;
overlay.ToolTip = string.Format("Zone from {0} to {1}", zone.DepthTop, zone.DepthBottom);

MultiBinding multibinding1 = new MultiBinding();
multibinding1.Converter = new AverageConverter();
multibinding1.Bindings.Add(new Binding("DepthTop") { Source = zone });
multibinding1.Bindings.Add(new Binding("DepthBottom") { Source = zone });
BindingOperations.SetBinding(overlay, ValueOverlay.ValueProperty, multibinding1);

MultiBinding multibinding2 = new MultiBinding();
multibinding2.Converter = new DifferenceConverter();
multibinding2.Bindings.Add(new Binding("DepthTop") { Source = zone });
multibinding2.Bindings.Add(new Binding("DepthBottom") { Source = zone });
BindingOperations.SetBinding(overlay, Series.ThicknessProperty, multibinding2);

Chart.Series.Add(overlay);