Thomthom's Selection Toys and Cleanup plugins may also be useful here.

thank you so much, this is great!

It works on a selection so you need to select some objects before running the code. Incidentally, I recommend that you do use a component for the marker, otherwise a raw geometry circle will have many 'parts' [face+all_edges] that with each return a 'center' ! Were there no messages in the Console? It probably said 'true' when it immediately finished processing the empty selection ! Try as I say and report what happens... It works for me. here's the line with the 2 'center' methods added... m=Sketchup.active_model;a=m.selection.to_a;m.start_operation('x');a.dup.each{|o|;g=m.active_entities.add_group();e=g.entities;(a-[o]).each{|i|e.add_line(o.bounds.center, i.bounds.center)}};m.commit_operation