You would need to use ?wavex and ?wavey instead of ?exprx and ?expry (the expressions would need to be strings).
However, there appears to be a bug - for me, with IC615 ISR16, it worked fine if I created the expression and used the "Reevaluate" icon to plot the results. However, if I ran a simulation, the simulations remained "running" and there was a strange MPS error in the job log file and it never returned.
I filed CCR 1138290 for this issue. If it's what you're seeing, please contact customer support and get a duplicate filed so that you can track the fix yourself.