Trouble using the secondary axis with a pivot chart

Hello,

I am having some issues implementing a secondary axis with my pivot chart. I am using two calculated fields for the months of May and June respectively. I need to implement a static horizontal line on the fill rate chart at a fixed value that stays as I use a slicer. Every time I try to format the data series, the secondary axis is grayed out.

If possible, I'd like to keep the current format and layout of the chart. I simplified the file as much as possible to save the time of looking through a large file.

Any help is greatly appreciated. Thanks.

Marc

Thanks Andy, that's exactly what I needed. Cheers.