Currently we use it on percentage based data. So it is simple to specify (100%) or some other hard coded value.

However there are situations where current values and limits are both pieces of data provided in the time series (HAProxy session limits, File descriptor limits, etc). If they y_val argument could be of type Number and not just Scalar we could create this sort of instantiated dynamic forecasting without having to go back and send the data itself as a percentage when creating the alert.