Although not as fancy as some of the commercial API's available for ASP.NET. Google's chart API is free and allows you to add charts to your web page. Follow the link in this blog to a great article on using the google chart api in .NET
Below is a link to a great article on using the google chart api with downloadable source for the ASP.NET control that wraps it.
http://pietschsoft.com/post/2007/12/Google-Charts-API-an-ASPNET-20-Server-Control.aspx