Dinamic variable in base of filters

[#1]

Hi, I've a graffic where I show the Amount of the last 6 month. I've got 6 variables, like this:

LastMonth = Date(monthstart(Today()), 'MMM-YYYY')

LastMonth1 = Date(monthstart(Today()-1), 'MMM-YYYY')

LastMonth2= Date(monthstart(Today()-2), 'MMM-YYYY')

LastMonth3 = Date(monthstart(Today()-3), 'MMM-YYYY')

LastMonth4 = Date(monthstart(Today()-4), 'MMM-YYYY')

LastMonth5 = Date(monthstart(Today()-5), 'MMM-YYYY')

And my Graffic is like this:

Product	=lastMonth5	=lastMonth4	=LastMonth3	=LastMonth2	=LastMonth1	=LastMonth
Product1	sum({$<Month={'$(LastMonth5)'}>}Amount)	sum({$<Month={'$(LastMonth4)'}>}Amount)	sum({$<Month={'$(LastMonth3)'}>}Amount)	sum({$<Month={'$(LastMonth2)'}>}Amount)	sum({$<Month={'$(LastMonth1)'}>}Amount)	sum({$<Month={'$(LastMonth)'}>}Amount)

I've got 6 expressions for this graffic:

sum({$<Month={'$(LastMonth)'}>}Amount)

sum({$<Month={'$(LastMonth1)'}>}Amount)

sum({$<Month={'$(LastMonth2)'}>}Amount)

sum({$<Month={'$(LastMonth3)'}>}Amount)

sum({$<Month={'$(LastMonth4)'}>}Amount)

sum({$<Month={'$(LastMonth5)'}>}Amount)

The actually Graffic it would be that:

Product	may-12	Jun-12	Jul-12	Ago-12	Sep-12	Oct-12
Product1	100	500	600	485	458	711

I want to use, in the variable, the max date that I've got in my filters, not Today(). If now I filter for the Date 20111230, my graffic should be this:

Product	Jul-11	Ago-11	Sep-11	Oct-11	Nov-11	Dic-11
Prodcut 1	45	89	0	54	66	12

Then, I need to use in my variable LastMonth the Max. Date that I have in my filters:

LastMonth = Date(monthstart(Today()), 'MMM-YYYY')

LastMonth = Date(monthstart(MaxDateInMyFilter()), 'MMM-YYYY')

How Can I do that?

Thank you very much