In this blog post I will answer why PowerPivot Diagram View is missing for some users and how to fix it.
Below you can see Home Tab with Diagram View using Excel 2013. In Excel 2010 things are a little bit more complex. PowerPivot has two versions for Excel 2010 version 1 (SQL Server 2008 R2) and version 2 (SQL Server 2012). Version 1 simply doesn't now have Diagram View which has been introduced in Version 2. So to fix it you just need to install v2 which you can find on Microsoft website http://www.microsoft.com/en-us/bi/powerpivot.aspx (Remember you might have to do some windows updates as I believe it requires .net framework 4.0)