Calculate disk space growth over time

[jackpal]

I am providing summarized reports on disk space over several hosts using this query:

index=os sourcetype=df host=host1 OR host=host2

| eval CPD_Disk=case(
filesystem LIKE "%gas%", "Gas Volume",
filesystem LIKE "%cadbas%", "CMS Volume",
filesystem LIKE "%spg%", "SPG Volume",
filesystem LIKE "%gen%", "Generator Volume",
filesystem LIKE "%stm%", "Steam Volume"
)
| chart eval(sum(UsedMBytes)/1024/1024) as TerraBytes by CPD_Disk| addcoltotals TerraBytes labelfield=CPD_Disk label=Total

I would like to provide the total amount of growth over the past 30 days. How could I add something like this ?