Git commits in Moodle 2.4.8 by James Pratt

commit 5267a0519133ca447f8d56c1d2aa0eb158ede0bf
Author: James Pratt <me@jamiep.org>
Date:   Wed, 27 Nov 2013 13:49:19 +0800
Tag:    v2.4.8

MDL-43079 quiz statistics : fix stats graph when all grades zero