I suspect you are trying to trace the current velocity when you spin your motor (configured as rotary axis) and you want to see exactly RPM value,
so if I’m wrong forget my comment 
, but if I’m correct you have to add a formula on the graph you have when you trace ParID 251 as described by Christoph.
Regarding a new curve with formula based on existing signal you can follow the guidelines described by Michael in his post Guide: Work with mapp Cockpit - Share Info & Ideas / How-To & Wikis - B&R Community
Ciao
Valerio