Hi,
The lastperiods function can only accept positive first parameter (which indicates the number of periods to include in the calc (as opposed to the previousperiod function which can take negative arguments). if you tried to use this function in a visualization that is ordered descending by time, but would like to sum the next n periods (which would be the last n if the ordering was ascending) you simply can do it. (Of course there is a workaround by creating a calculated column.)
Thanks,
Laszlo
Thanks for pointing this out. Please consider using NavigatePeriod as well.