This must be a world record: gnuplot has had the "--with-boxes" option 
for more than 10 years, and this was never documented.
Fixed in git.
-------------------------------------------------------
   Riccardo (Jack) Lucchetti
   Dipartimento di Scienze Economiche e Sociali (DiSES)
   Università Politecnica delle Marche
   (formerly known as Università di Ancona)
   r.lucchetti(a)univpm.it
   
http://www2.econ.univpm.it/servizi/hpp/lucchetti
-------------------------------------------------------