Getting error while installing SP Userpoll webpart

Jan 19, 2010 at 10:49 AM

Getting following error on installing SP Userpoll webpart

Parser Error Message: Could not load file or assembly 'System.Web.DataVisualization, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.
 what should be the issue?