Calculating rate of change from gauge

Hi,

I am graphing barometric pressure, with the graph showing a two day period. I would like use gprint to add the rate of change over the last hour to the graph. I would like to take the value from 1 hour ago and subtract the LAST value. The trouble I am having is getting the value from 1 hour ago.  Below is what I am trying to do:

VDEF:last=pressure,LAST GPRINT:last:"Current=%4.2lf%s" \
CDEF:old=pressure,LAST-1h
VDEF:ROC=old,LAST,- GPRINT:ROC:"Change=%4.2lf%s" \

But I am unsure how to get the value from 1 hour ago. which is what I was hoping "CDEF:old=pressure,LAST-1h" would accomplish.
Thanks in advance for any help,

Jeff