> not sure what would be a good UI for switching an element to relative positioning

  1. Right-click on an existing object, offer drop-down context menu.
  2. Menu item `Position relative to...`.
  3. The cursor now selects _other_ objects in the field. 
     a. If there is no other object, then offer to create a new label-less node with (x,y); default to the origin.
     b. Once an object is selected, then offer `right of`, `left of`, `north of`, `south of`, `southeast of`, etc as a drop-down menu, and a field for radial displacement.
        i. As a stretch goal, offer a `Custom position...` button to specify an (x, y) displacement, or a polar angle and radial displacement. These three options (fixed offsets, Cartesian, polar) could also be tabs in the resultant menu from (b) above.