Accumulate sales for months

[#1]

I have the sales for each year en months, How I accumulate the sales for Months, for example January $ 1000.00, Feb 3000.00, Mar 2000.00, in a simple table.

Vendor     Month     Year 2012    Accumulate

XXXXX          Jan          1000          1000

                   Feb         3000          4000

                   Mar         2000          6000

Thank A lot

[#2]

Hi.

Where I can Find a manual about make expressions for example

sum(if (InYearToDate (, and more command

Thank alot