Unfortunately, the axes object has only two values for the [x|y]AxisLocation property -- Top/Bottom; no Origin choice is available.
Use a second axis whose location is at the origin or draw it manually are only choices. I think the first is doable ok w/ some playing to turn off all the extraneous stuff you won't want; there's an example of using a second axis (altho for different purpose it'll still give you the idea of how to manipulate it) in the section on enhancing graphics.
Well, I tried--I could overlay the second axes object as desired and show the xticks at the y-origin of the first but couldn't ever figure out how to get it to not obscure the bars of the bar plot as long as the 'visible' property is 'on'. Of course, if set it to 'off', then the axis disappears, too. :(
Looks like drawing the line and using text() to label it is probably easiest. Wrap the commands in a utility function to call if doing this often.
I think it's a reasonable enhancement request for the 'Origin' keyword for the axis location.
