Assigning a series to a secondary y axis when it is buried in the chart

I have a chart with stacked data series, with thousand of points on the x axis. I want to add a few lines from specific data series and assign them to the secondary axis. But the series (actually a vertical line) are hidden in the stacked areas and I have no way to graphically point to and select it with the mouse to assign it. It’s really a very small needle in a huge haystack.
Is there another way to find and select a given series, other than graphically in the chart?

Without a realistic example I cannot even understand in what way a second y-axis used with a stacked chart should be useful or even justified.

Go to one of the source cells for the series under discussion, save its value or formula, and change the content to an exceptional different value then. Now the needle in the haystack should have become a pitchfork. You may pick the series with a grip to the fork, and call the dialog Format Data Series... for it.
Editing done, restore the previous value/formula for the chosen cell, re-needle the data point this way, and lean back.

Excellent suggestion, I don’t know how I didn’t think of that. Thanks a lot.
FYI, I use the second y-axis to display another function that is not part of the stack. It still (I believe) has to be an area chart, but I can display it as a line graph, with the same categories, independently of the stack.