How to fix a vertical line in the center of the chart


I have a fixed vertical line that I called "focusLine".
"layout" is the x:name of the AbsoluteLayout in XAML.

var focusLine = new BoxView { Color = Color.Black };
            layout.Children.Add(focusLine,new Rectangle(0.5, 0, 1, 1),AbsoluteLayoutFlags.PositionProportional|AbsoluteLayoutFlags.HeightProportional);

I set the x position of new Rectangle to 0.5 that means vertical line appears in the middle of the AbsoluteLayout. Instead, I want to set the x position aligned with the center of the charts. See attachment please.

May you help me please?