Moving a Series on the Secondary Axis to the Background

[dflak]

The attached spreadsheet has a table with a list of people and buckets for how long it takes them to do certain tasks.

I have the table data plotted on a clustered column chart, and since the average number of days dominates the numbers, I put it on the secondary axis. The I adjusted the overlap and gap so the bar for the average is as wide as the people on the X-axis.

As presented, there is NO fill and a solid black outline. This is acceptable. However, I'd like to show it behind the other bars with some shading. I was wondering if there is a way to accomplish this.