The problem with a shortest path algorithm is that it ends up either with the lines intersecting each other, or looking like a chaotic mess.
See particularly the example here: http://stackoverflow.com/questions/14263284/create-non-intersecting-polygon-passing-through-all-given-points

Instead of throwing such a huge investment into what is essentially an advanced AI sub-routine, we instead plan on adding an auto-snap capability, as we have when drawing pipe -- that will save having to click on each plant.

--J