How to set the reference zone in XY-plot
 

I have a rotating region in my case. and I want to plot the relative velocity. How to choose the reference zone?

There is one field function for the relative velocity in each reference frame. You can choose it like any other field function.

Hi, thank you for your reply. I'm sorry I am not understand clearly...

actually I defined a line in the domain, then choose plot->XY Plot->relative velocity... but I can not find any option to change the reference frame of this curve.
can you show it one step by step? thank you so much.

In the dialog where you define the quantity to display, you can just choose the velocity relative to a reference frame, just like any other field function. Don't look for "relative velocity" but for "velocity". The velocities relative to a reference frame could be found when pointing on the little black arrow.

Thank you so much......:)