Parameters
http://thepebkac.wordpress.com/2011/11/09/tableau-parameters-and-calculated-fields/
http://www.alansmitheepresents.org/2011/03/changing-viz-types-with-dashboard.html
http://community.tableausoftware.com/thread/109920
alternatively, hide using action filters
http://community.tableausoftware.com/thread/117501
URLS and Parameters for Reports
http://www.tableausoftware.com/support/forum/topic/url-parameter-report-pdf-format#comment-44996
http://www.tableausoftware.com/support/forum/topic/passing-filter-parameters-url