this works (function last): grpavg[<group>,<item>,last,5m]

other functions (tested sum, avg) doesn't work:
grpavg[<group>,<item>,avg,5m]
grpavg[<group>,<item>,sum,5m]

but works:
grpavg[<group>,<item>,avg,300]
grpavg[<group>,<item>,sum,300]

Simply unit symbols doesn't work with all functions in aggregate items