If I have understood your post correctly I believe you have data that shows your sales figures as a cumulative balance for each week but wish to see it as a weekly total plotted against the week number.
One way to achieve this is to use the RangeSum function.
Set up a straight table with Week as your dimension. Open the properties of the table and in the expression tab add two expressions:
Sum ([Total Sales])
Sum([Total Sales])-RangeSum (above(Sum([Total Sales]),-1,Count(Week)))
The first expression is a straight forward summation of sales values. The second expression does two things. Firstly, it calculates the total sales for the previous week. Secondly, it subtracts this value from the total sales for the current week resulting in a weekly balance. It repeats this process for each week value listed in your dimension and so populates your table with weekly sales values.
For this solution to work correctly the data must be listed as in the table above with most recent week at the top i.e week sorted descending by week number.
Hope that helps.