all 2d objects in Onshape (& solid works) "live" in what is called a "sketch plane" so to make the plumb bob line I would need to make a sketch plane then draw on that to make a vertical line. so any of the guide lines you see above need to live in a sketch plane. so you cannot really have 2d lines floating around in the actual 3d model..
the reason for this is Mechanical cad can go from actual model to fabrication (think cad/cam) so they do not want to have all this debris floating around in the 3d universe..