Vertical line in a histogram (vertical bar chart)
 

I have a dynamic hystogram chart that keeps track of current process
performance. So it is always changing as new data is entered.
I would like to have a vertical line indicating the average. (And if I get
that figured out I would add more vertical lines representing UCL and LCL
control limits)
I have tried methods using a secondary axis and error bars, but they don't
quite work because the scale for the secondary category axis and the primary
category axis are not the same, so the line is in the wrong place.
I could set the scales equal manually, but that doesn't quite work for me
because the charts are always changing and I need the scales to move
automatically.
Any help on this would be awesome.
Thanks so much!