I want to add a vertical line to a histogram, which is the whole height of the diagram. For example, this *almost* works.

use http://www.stata-press.com/data/r13/auto, clear

//Histogram with a vertical line at 15, but need to know the right height
histogram mpg, fraction addplot(pci 0 15 .35 15) legend(label(2 "My Threshold"))
The problem is that the pci requires giving the height, and I have to eyeball it to .35. Is there a way to do this automatically, so that I can get a line the full height of plot? Obviously, I can tweak this for a given graph, but I am generating dozens of histograms of this type and it would be much easier to do it automatically.