I'm not at all sure what you intend that last line to do: but since it starts at the right hand side and goes further right from there it won't be at all visible within the rest of your drawing:
grfx.DrawLine(Pens.Black, Rectangle.Right, Rectangle.Top, Rectangle.Left+Rectangle.Right, Rectangle.Bottom);
Since both left and right will be positive, left + rigth will be greater than right alone (unless left is zero, in which case it's a vertical line you've already drawn).