Corners are only present in three-dimensional chart types.
Changing properties: To change the properties of the corners, select one of them, right-click it and choose the command Corners: Properties from the context menu.
Tip: Alternatively, you can choose this command via the Edit properties icon 
on the ribbon tab Chart | group Chart elements.
You can make the following settings:
3D view tab
Here, you can set the 3D effect for the entire chart. For more information, see Chart properties, 3D view tab.