Dear Didier,
I cannot seem to get Option 4 to work (drawing a line perpendicular to a face through a point). It draws a temporary line through the centroid, but it won't let me select a point of enter a length. Your perp_line_plane.rb works just fine, and I assumed you had integrated that script into the new collection (hence the 13th icon). Ah, that's it! The 13th Icon!.
I like the way your perp_line_plane.rb works: select surface/face and cpoint (on face), then select 'Line perpendicular to face' from Plugins. Enter length in dialogue box and 'bingo', the line is drawn. Beautiful!
Kind regards,
Bob