Scaling Third Axis in Chart

Hey,

I have a chart that displays stacked bars. The X-axis is two-levels with months and names. The Y-axis is percentage. I have a secondary axis. It has dates shown as lines with markers on the same X-axis as the stacked bars.

I'd like to draw a horizontal line across at 60%. From what I've read I should have a two points in a new series one at 0, 0.6 and one at 1, 0.6. Fomatted as a scatter plot with smooth lines. I get the line, but here comes my issue. It's a third X-axis. I need to scale that third axis so 0 is the minimum and 1 is the maximum.


But I have no idea on how to access this third axis and scale it properly so the line extends all the way across. I can't use the axis used for the primary and secondary because they're both the two-levels month and name. Anyone know how to do this? Or a better way?

I'm trying to do this in VBA so I can automate table + chart generation. So manually drawing a line won't work.

Thanks!