Charting control in ASP.NET 3.5 greyed out.

I have a webpage built in ASP.NET 3.5. The Charting control is greyed out in the designer tool box.

I have a line in the web.config

<add tagPrefix="asp" namespace="System.Web.UI.DataVisualization.Charting" assembly="System.Web.DataVisualization, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

How can I enable the control?