st: Adding lines to multiple histograms

["MA V" <ma_v@hotmail.com>]

Dear Statalist members,
I am drawing multiple histograms in one plot by using the "by" option.
e.g. hist age, by(city)
(note: I have 12 cities)
My question is the following: is it possible to add one vertical line to 
each individual histogram(note that these lines have different values for 
different cities, i.e. for different histograms)?