I am writing a WPF chart and use Visuals for performance. The code looks like:

public class DrawingCanvas2 : Canvas
{
    private List<Visual> _visuals = new List<Visual>();

    protected override Visual GetVisualChild( int index ) { return _visuals[index]; }
    protected override int VisualChildrenCount { get { return _visuals.Count; } }

    public void AddVisual( Visual visual )
    {
        _visuals.Add( visual );

        base.AddVisualChild( visual );
        base.AddLogicalChild( visual );
    }
}

Beside DrawingVisual elements (line, text) I need a ComboBox in the chart. So I tried this:

    public DrawingCanvas2()
    {
        ComboBox box = new ComboBox();
        AddVisual( box );

        box.Width = 100;
        box.Height = 30;

        Canvas.SetLeft( box, 10 );
        Canvas.SetTop( box, 10 );
    }

but it does not work, there is no ComboBox displayed. What I am missing?