System.Web.UI.DataVisualization.Charting missing error in .net 4.0?

When I upgraded my website from .net 3.5 to 4.0, some of the references in my web.config didn't automatically get updated to 4.0. After I manually updated them, everything worked.

Click on Add References in your project and in the .NET Components tab add the System.Web.DataVisualization. This should solve your problem.

Click on Add References in your project and Browse to C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0 and add the System.Web.DataVisualization dll file.

Before adding the dll remove the previous one and make sure that change the Target Frameweok to V4.0 in the project properties.

This will solve your problem.