arrows in histograms

Dear all,

I have a null frequency histogram of  pairwise associations values.
How can I put  an arrow on the histogram to indicate the critical value
obtained from the null distribution? [In this case, quantile 0.05].

Thanks

Rog??rio R. Silva
MZUSP http://www.mz.usp.br
Linux User # 354364
Linux counter http://counter.li.org