How to plot average pressure from multiple monitoring points in Creo Flow Analysis XY Plot?

Hi,

I am trying to plot the average pressure from several monitoring points in Creo Flow Analysis / Creo 12.

I have 10 monitoring points named

Point 01, Point 02, ... Point 10

According to the documentation, I understood that monitoring point values can be accessed using syntax like:

flow.P@probe."Point01"

and that user-defined XY plot variables should be created using:

plot.varname

So I tried defining an expression like:

plot.P_avg = (flow.P@probe."Point 01" + [...] + flow.P@probe."Point 10")/10
#plot.P_avg: Average Pressure [Pa]

However, I cannot find plot.P_avg as a selectable variable in the XY Plot panel or elsewhere afterwards.

Has anyone successfully plotted an average value from multiple monitoring points in Creo Flow Analysis?
Am I missing a specific place/context where the expression must be created, or is there another syntax required?

Thanks in advance.