WinRT DataBinding with ObservableCollection

Question

In my ViewModel I've defined following ObservableCollection.

    private readonly ObservableCollection<ChartSerie> _seriesData = new ObservableCollection<ChartSerie>();
    public ObservableCollection<ChartSerie> SeriesData
    {
        get { return _seriesData; }
    }

On my Page I bind this collection.

ItemSource="{Binding SeriesData}"

ItemSource is a DependencyProperty in my control.

public object ItemSource
{
    get { return (object)GetValue(ItemSourceProperty); }
    set { SetValue(ItemSourceProperty, value); }
}

public static readonly DependencyProperty ItemSourceProperty =
    DependencyProperty.Register("ItemSource", typeof(object), typeof(MultipleColumnChart), new PropertyMetadata(null, OnItemSourceChanged));

The control inherits from Grid. I've defined OnItemSourceChanged as following.

private static void OnItemSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{            
    var chart = d as MultipleDoubleColumnChart;
    foreach (var serie in (IList)chart.ItemSource)
    {
        var child = new ColumnChart();
        ...
        ...
    }
}

When the Page and ViewModel are created the SeriesData collection is empty and the first and only call to OnItemSourceChanged does nothing (as expected). After a certain button click on the page the data were loaded and new ChartSerie object were created and added to SeriesData.

Although SeriesData is an ObservableCollection the OnItemSourceChanged Method is not called anymore.

What is/went wrong and what do I need to change that OnItemSourceChanged will be called?