Can not use calculator to plot figure

I was trying to plot the rise time of a waveform that I got from a
simulation using the calculator function. However it didn't work and I
got a warning "Evaluation error (unknown scalar data type:nil)
Expression: error)". I used to do this at another machine and it
worked. Can anyone tell me how to fix this problem? Thanks.

 

Hi Allen,

To be entirely honest with you, I hardly believe this kind of error is
hardware/software dependant. You may need to cross-check your
expression again and I'm pretty much sure you will find a little
naughty thing that prevents you from plotting your rise time.
Otherwise, it would be handy if you give a more detailed description
of the steps you're following to end up with this error.

Riad.