How to define the top axis which is a function of the bottom axis in Igor.

Hi guys

I remember I did this myself before, but it has been quite some time I haven't used Igor and I forgot how to do it now.

Can someone just tell me briefly? I remember it has something to do with the transform axis, but don't remember clearly where to find it and how to define the axis myself!

Cheers,
Choose Graph->Packages->Transform Axes and click the Help button in the resulting control panel.