how to generate a planned vs actual curve basis on duration ?

By definition to draw a curve you need two axis.  Typically in planning these axis are time and resource quantity/cost.

if you want to use time for both axis, you need to add a resource that equals the time.  When you add a resource to an activity the default quantity is ususally 1h/h..... Therefore you need to add the same resource to every activity in the plan.  That means the resource qauntity will equal the duration.

All P6's resource/earned curves will be based on the duration of the activities.

thanks.......got it