The type or namespace name 'UI' does not exist in the namespace 'EPiServer'

This morning I updated an EpiServer project from Visual Studio 2008 to 2012. The only problem I had was this error when building:

The type or namespace name 'UI' does not exist in the namespace 'EPiServer' (are you missing an assembly reference?)

I discovered that Episerver.UI.dll has some dependencies that I didn't have. The solution was install the Microsoft Chart Controls on my machine