Is the attached something like what you're looking for? The two key bits to this are: filtering for non-Null values in the table calculation - see this post for details: http://community.tableau.com/thread/110734 and setting the date filter appropriately.
If you need more help, I suggest posting a packaged workbook with some sample data.
Awesome - was just needing to figure out how to do this and its already posted - love it!!!!
You're welcome! The forums are really a treasure trove of information. After getting started with the online Tableau training videos and the "Rapid Graphs in Tableau" http://www.amazon.com/Rapid-Graphs-Tableau-Software-Actionable/dp/B004NFC1KA e-book, most of my Tableau learning has either been from working through the posted answers or trying to answer forum posts myself.