populate pie chart using Excel spreadsheet

[FirstMM]

Hey guys, I am a complete Visio newbie so please go easy on me!

I have spent the last hour trying to "link" a Visio pie chart to an excel spread sheet.
I have added the data source successfully but cannot get anything except equal slices - the external data source (Excel) is being completely ignored by Visio as far as the pie chart is concerned.
It does, however, show the data in the "External Data" window.

I haven't been able to find anything on-line about this....  Am I barking up the wrong tree here?

I want to be able to edit the spreadsheet and have the pie chart automatically update.  The Visio pie chart will be permanently displayed on a screen so I don't want to use Excel and have the raw data displayed every time I update.

[FirstMM]

btw - I am using Visio Professional 2013