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)
This will upgrade your PowerPivot and your models so ensure everyone you work with has v2 as well!
Important: Version 1 is limited in functionality so I strongly recommend to upgrade to version 2
Alternatively you can upgrade your Excel to version 2013 but you also need to ensure people you share PowerPivot model with have Excel 2013 as well.
I hope this blog post clarified what Diagram View is missing and how to solve it.