How to calculate balance differences between months in chart with time dimension

[adam1988]

Hi,

i'm trying to solve this but I dont know how, so I wondering if I'm that stupid or what...

I've got data about customers and balances on their accounts at every last day of the month. It looks this way for example:

period	customer	balance
11-2012	John	255
10-2012	John	240
09-2012	John	180
08-2012	John	150

Users scenario is simple. They filter one PERIOD and see SUM OF BALANCES from all customers. Also, they need to see how this SUM OF BALANCES went throw the time in chart. I use PERIOD as dimension and exppression "sum({$<period=>} balance)" - there is no problem.

From table I can see Johns' amount of money between 08-2012 (150) and 09-2012 (180) increased so the diference in 09-2012 is 30. in 10-2012 is 60 and 11-2012 is 15.

How can I calculate these differences? For one selected month, it's easy "sum({$<period=LastPeriod>} balance)" where LastPeriod = addmonth(period, -1). But how to calculate/add column with DIFFERENCES into previous table? AND primarily, how can I present these differences in chart with period dimension?

Because if I use "sum({$<period=>} balance)" I can't use set analysis for period again.

Is it even possible?

Thx. Adam