Improve the plotWidget

Summary:
D10786 but on atcore base. + Authors updated.
This is to keep the commit history clean since they are on different bases.
This will be merged in to atelier as well when Landed.

Reviewers: tcanabrava, laysrodrigues, patrickelectric

Reviewed By: laysrodrigues

Subscribers: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D11770