[rrd-users] calculate change / graph change between last and last-1

[John Stile]

I have a graph which shows the size of my server backups.

I would like to add to the ledgend (and maybe graph) the change in size
from the last backup.

Do I define a vdef for size,LAST and a vdef for LAST-1, 
then calculate a cdef of the difference
then a vdef of cdef of the difference, 
and finally a gprint of the vdef?

How do I get LAST-1?

Is there a better way?