Well, it's possible to some extent.. You can have Visio diagram in sharepoint auto-update from excel data when you change some values in excel sheet for shapes which already exist on the daigram (like name of a person for example). But you can't make sharepoint or visio auto-generate diagram when you add or delete persons in excel file, or when some person changes it's position for example, i.e. the functionality to automatically add/delete or move the shapes on diagram depending on the data in excel does not exist.
Also note stackoverflow is basically focusing on questions related to programming in some way, so you may try other places.