I did some checking and we don't have a workaround for this. I'll make sure the PM and Development look at it and add as a feature request.
+1 for this request, and the general ability to alert on traffic deltas.
In our case, we want to alert if an interface's traffic percentage changes, say, drops in half.
I've been searching for an update to this topic. Was a workaround ever identified? If not, was this ever added as a feature?