horizontal lines on stripchart

Hi.

I would like to add a horizontal line to each column of data on a 
stripchart (vertical=T) to indicate the mean(rather like the package 
Prism does - for those that have used this) for each dataset. I presume 
the best way to do this would be with "lines" but after much trying  / 
playing about I can't figure it out. Can anyone help?

Thanks