The 2d Constraints tools are based on a 2d host plane when solving.
It would be good to be able to display the host plane in the Properties Panel and also in the View Window on demand (Right-click option when the host plane is selected in the PP)
This would help the user understand and 'debug' his constraints.
Exposing the host plane would also pave the way for better and more reliable constraints between:-
1. 2d elements, some of which may be on different non coplanar planes,
2. 2d and 3d elements, and
3. Nested elements in Cells.
Listing the host plane (named) in the Properties Panel would also allow the user to assign the host plane if necessary, instead of guessing what the solver has done.