Replace deprecated QChart calls.
Summary: Newer Qt have deprecated a few members of QCharts. Update our graph to not use them anymore
Reviewers: patrickelectric, laysrodrigues, tcanabrava
Reviewed By: patrickelectric, tcanabrava
Subscribers: Atelier: AtCore
Tags: Atelier: AtCore
Differential Revision: https://phabricator.kde.org/D17541