[collectd] snmp plugin and simple math

[Sergey]

Hello.

I have such a problem: hardware have two OIDs "full memory"
and "avaliabe memory". I would like to display "used memory".
I think this can be done using the perl plugin or custom rrd
grapth, but it is more interesting to modify the data when
writing to rrd. 

Something like

Values ".1.3.6.1.4.1.2021.4.5.0" ".1.3.6.1.4.1.2021.4.5.0-.1.3.6.1.4.1.2021.4.11.0"

Is this possible?

-- 
Regards, Sergey