[Jprogramming] Add vertical line to plot

[Ryan Eckbo]

I have a plot of a histogram, i.e.

  plot bins; counts

to which I'd like to add a vertical line denoting the mean.  I saw a 
post on how to
create grid lines using complex numbers, but I don't know how to add a 
vertical line
to an existing plot of data.  Does anyone know how?

Thanks!
Ryan