Excel Graphs - aspect ratio

[Vidcapper]

When I set up graphs, they seem to default to a 4:3 ratio on-screen. I
would like to expand their width to take advantage of my wide screen,
but the obvious method of trying to drag side-bars doesn't seem to work.
Am I missing something obvious?

 

[Cexcel]

have you tried from the corners?

 

[Vidcapper]

have you tried from the corners?

Yes.