Add additional label at top of Y-axis in Combo chart

[staceyzhang]

Hello, 

I have a question about adding a label at the top of Y-axis in a Combo chart. Since the unit of Net sales (right side of Y-axis) is in thousands, we want to add a label at the top of Y-axis which is easy to review (like the red box in attached image). Could someone help with me?

 Thank you!