Shane Miller found interesting hidden feature in workout mode in Zwift https://www.youtube.com/watch?v=KC5RWGrDE_s&list=PLeDc5AkDnMO-3fewHlDVN0eYflRParDSv&index=11
It should not be hard to implement adding flag show_avg=“1” to .zwo files. Please do it 
Cool. Thanks for pointing that out! We can add that to our plans.