Semi additive measures in VA

[FrankE]

I have a dataset that includes several periods of data with a period end balance on each record.  My VA report has a period slider at the top that I can use to select the date range for the report.  I'm looking to display the current balance as of the upper bound of the date slider.  I believe this would be considered a semi-additive measure?  Can someone point me in the direction as how to handle this?

Example:

CustomerID      Period       Balance

1234                 Jan2001    $300

1234                 Feb2001   $100

1234                 Mar2001   $300

Just using the default aggregation, the field would show the sum of the rows included by the slider.  What I want is the balance as of the end of the slider.