problem with defining plots
 

hi
I'm working on a ventilation project on fluent and now I want to define plots like y/Dh-u/umax . y/Dh for one axis and u/umax for another axis .but I don't know how. I use custom field function but there isn't parameters like Dh(hydrulic diameter) or maximum velocity on that menu
can anybody help me ?
I apologize for my weak english and appreciate for any help