Why are ActualWidth and ActualHeight 0.0 in this case?

ActualHeight and ActualWidth are not set until the control is measured and arranged. Usually there is nothing in InitializeComponent() that causes a measure, so when it returns these will still be zero.

You can force these to be computed earlier by simply calling the window's Measure() and Arrange() methods manually after the window's InitializeComponent() returns.

If you are sizing to content:

window.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
window.Arrange(new Rect(0, 0, window.DesiredSize.Width, window.DesiredSize.Height));

If you are using an explicit window size:

window.Measure(new Size(Width, Height));
window.Arrange(new Rect(0, 0, window.DesiredSize.Width, window.DesiredSize.Height));

Ray is correct (+1) that this is due to the fact that the measure and arrange pass has not executed yet. However, rather than force another layout pass (expensive), you can just wait until your control has loaded before accessing the ActualXxx properties:

public MyWindow()
{
    Loaded += delegate
    {
        // access ActualWidth and ActualHeight here
    };

}

In our case the solution was simple, as everybody said ActualWidth and ActualHeight needed to get called after the Loaded even completes, So we just wrapped the code in a dispatcher and set the priority to Loaded as below:

Dispatcher.Invoke(new Action(() =>
{
   graphHeight = ActualHeight;
   graphWidth = ActualWidth;
}), DispatcherPriority.Loaded);