Fixed grades issue for progress page graph.
TNL-1022
Merge pull request #647 from edx/waheed/tnl1022-fix-grades-issue-for-progress-page-graph

Fixed grades issue for progress page graph.
Fixed grades issue for progress page graph.
TNL-1022
Merge pull request #648 from edx/hotfix/2014-12-17

Fixed grades issue for progress page graph.
Working grade override.
Added data checking.
Tidy up.