[metro]

I have my data organized as date in column A, amount in column B and category
in column C, e.g. 3. Aug 2009 spent $ 200 on clothing and 4. Aug 2009 spent
$50 on restaurant.
I would like to keep track of montly spending on each category and the total
of each month.
My problem is that the date dimension doesn't seem to be possible to group
to a higher level than on the actual date from the data level.
How do I do it?