Please add some snap line construction tools: A horizontal line that is dragged out of the horizontal ruler, and a vertical line that is dragged out of the vertical ruler. These construction lines are snapped to by blocks and lines on the edges, centres, and line connection points to allow controls to be quickly aligned. You should be able to use more than one of each at the same time.
Note that you can currently draw horizontal and vertical lines by pressing Ctrl while you move/drag the mouse. Isn't it helpful already?