Calculate total amount of transfered data

[Ammer]

Hi,

I am having trouble creating a calculated item.
Mainly because i suck at math

I have got a snmp query that receives the current bytes in from a network device.
By mean of delta, i calculate the data speed the interface is using.

Now i want to calculate the total amount of data that goed through the interface.
I gues i need to create a calculated item for this, but i havent got a clue what it should look like.

Anyone here know how to do that?

Thnx

[Ammer]

I got a little further,
i made the following caculation
sum("item_key",2592000)
This is the the value of the key added up for 2592000 seconds.
However, this gives me a more or less straight line.
Is there a way to make it start at 0 eacht first day of the month?
So i can het graphs that show the amount of data that is beging downloaded monthly?