Creating an average line for a 100% stacked column chart

[nacky]

I have created an average line for a 100% stacked column chart but I cannot get the line to span across the whole area. Typically you can achieve this on a normal column chart by un-checking 'values cross between categorys' for the secondary axis the line is on but this option is unavailable. Is there any other way to get the line to span across the whole area?

[Andy Pope]

You would put the line on the secondary axis.
Via Chart Options > Axes. Enable the secondary x-axis.
Format the secondary x axis to not cross between categories.
Remove tickmarks and tick labels.