Hi Pierre,
In our DashBoard samples we have used 'Syncfusion.SampleLayout.WPF.dll' for design purpose and GroupBarAdv is one of our control present in 'Syncfusion.Tools.WPF' dll. Please find the help link below for this control.
Link: http://help.syncfusion.com/ug/wpf/tools/default.htm#!documents/groupbar.htm
Please let us know if you need further assistance.
Regards,
Jegan Raj M