Chart using week as Dimension and shows current and previous week's data

[#1]

Hi I am trying to find out if this is possible. 

WeekStart	Count1	Previous Week	Count2
1/1/2012	10	12/25/2011	3
1/8/2012	15	1/1/2012	10
1/15/2012	21	1/8/2012	15
1/22/2012	5	1/15/2012	21
1/29/2012	3	1/22/2012	5
2/5/2012	11	1/29/2012	3

I have tried as following

Dimension: WeekStart

Count1 :

=sum(Count)

Previous Week:

=WeekStart(WeekStart, -1, -1)

Count2:

=sum({$<WeekStart={$(=WeekStart(WeekStart, -1, -1))}>} Count) // -> not working as is

Do you know if above table is possible?

Thanks,

Sean

[#2]

Thanks,

I have modified the expression to

= if( RowNo() = 1, 0, Above(column(1)))