I am trying to bind a XamDataChart to an ObservableCollection so that it gets updated in real time as data is added.  Here's the XAML:

<ig:XamDataChart Name="AltitudeDataChart" >
<ig:XamDataChart.Axes>
<ig:CategoryDateTimeXAxis x:Name="altXAxis"
VerticalAlignment="Bottom"
DateTimeMemberPath="GeoPosition.Timestamp.DateTime" DisplayType="Continuous" ItemsSource="{Binding}" Label="{}">
<ig:CategoryDateTimeXAxis.LabelSettings>
<ig:AxisLabelSettings Location="OutsideBottom" Extent="25"/>
</ig:CategoryDateTimeXAxis.LabelSettings>
</ig:CategoryDateTimeXAxis>
<ig:NumericYAxis x:Name="altYAxis" >
<ig:NumericYAxis.LabelSettings >
<ig:AxisLabelSettings Location="OutsideLeft" FontSize="18" Foreground="{StaticResource PhoneForegroundBrush}"/>
</ig:NumericYAxis.LabelSettings>
</ig:NumericYAxis>
</ig:XamDataChart.Axes>
<ig:XamDataChart.Series>
<ig:AreaSeries x:Name="AltitudeAreaSeries" ValueMemberPath="GeoPosition.Location.Altitude"
ItemsSource="{Binding}" TrendLineType="CubicFit" TrendLineBrush="Yellow" TrendLineDashArray="5" TrendLineThickness="3"XAxis="{Binding ElementName=altXAxis}" YAxis="{Binding ElementName=altYAxis}">
<ig:AreaSeries.Brush>
<SolidColorBrush Color="Blue"></SolidColorBrush>
</ig:AreaSeries.Brush>
</ig:AreaSeries>
</ig:XamDataChart.Series>
</ig:XamDataChart>

In the code behind, I have the following:

this.AltitudeDataChart.DataContext = activityViewModel.CurrentActivity.Data;

where Data is an object of the following type: ObservableCollection<ActivityData> and ActivityData has the following property:

public GeoPosition<GeoCoordinate> GeoPosition { get; set; } ... parts omitted for brevity.

Suffice it to say that if I reference GeoPosition.Location.Altitude and GeoPosition.Timestamp.DateTime, the binding should work.  And it does work fine if I don't go to the control which is hosted in a Pivot control.  But as soon as an update is made and I'm on the chart, I get the following exception:

{System.ArgumentNullException: Value cannot be null. Parameter name: collection    at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
   at Infragistics.FastItemsSource.dataSourceAdd(Int32 position, IList newItems)
   at Infragistics.FastItemsSource.<Attach>b__0(FastItemsSource fastItemsSource, Object sender, NotifyCollectionChangedEventArgs e)
   at Infragistics.ItemSourceEventProxy.collectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.InsertItem(Int32 index, T item)
   at System.Collections.ObjectModel.Collection`1.Add(T item)
   at ActivityTracker.Code.Activity.<AddData>d__1.MoveNext()}

Does anyone have any ideas here?