"Delivered", really? To be honest that solution looks awfully strange. You suggest use "Power KPI" but 2 line charts look pretty ugly there - there are many missing features like no possibility to add markers, and no "customize series" in data lables (which is key feature for me). Instead of adding simple solution to line chart (what was asked) you actually did something weird (sorry - it's just the way I see it). Would be good if you can reconsider and add secondary axis to line chart after all.
Or maybe I am missing something and can explain how can I have secondary axis in line chart. Thanks.
Power KPI supports dual Y-axis now.