Bi-variate Map Example from Web

"The view below highlights the current issue with Tableau dual axis mapping visualizations--dots are stores, and filled areas are customer demand. Customer demand exists in areas (postal codes) where there are no stores, but these don't show up because they do not have a corresponding joined data point.  Want to show all stores and all area demand in one view.  This is the most requested view in our company and I would love to run it from Tableau."

 http://community.tableausoftware.com/ideas/1492