Assembly error trying to use DataVisualization - what does it mean?

I'm writing an ASP.NET 3.5 app, and I'm trying to add a chart to it. I added a reference to System.Web.DataVisualization to the project, but when I run it I get this error:

error BC30652: Reference required to assembly 'System.Web.DataVisualization, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' containing the type 'System.Web.UI.开发者_运维技巧DataVisualization.Charting.Chart'. Add one to your project. 

What is this trying to tell me?

Seems like the error is pretty clear what it wants. You've got a reference to something (another DLL) which requires you to also reference the System.web.DataVisualization assembly.

Never mind - the necessary web.config settings weren't in place. Not sure why adding the reference didn't set up web.config for me, but I just copied the settings from another project and it's working fine now.