[Applet]Fix traffic monitor labels width
Summary:
Use text metrics to set a proper space for labels.
And remove unused plotter width.
Test Plan:
Change default font styles and family.
Change small font size.
Space available must follow the font.
Reviewers: VDG, Plasma, jgrulich, ngraham
Reviewed By: VDG, jgrulich, ngraham
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27792