If you want to use charts in your web application, you should spend some time with Google chart api's. Very simple to use. I also found an open source project which implements the google chart api as ASP.NET server control.
Here is the link to the Google chart API.
Here is the link to the ASP.NET Google Chart Control.
I also found another interesting flash based open source chart project(OpenFlashChart) and here is the link to it,