Pivot Chart Select Data Source Buttons Greyed Out

I've created a new workbook and in it I created a new pivot table and chart.  When I right-mouse-click on the chart and choose Select Data then the Select Data Source dialog box opens.  However, only the Switch Row/Column button is active.  Under the Legend Entries section (on the left) the Add, Edit, Remove, Move Up, and Move Down buttons are all greyed out.  Under the Horizontal Axis Labels section (on the right) the Edit button is greyed out.

I'm using Office 2013 from my current Office subscription.  The workbook and worksheets were left at their defaults.  Protection was not turned on.  How do I enable editing within the Select Data Source dialog box?

Thanks in advance,

Andrew