[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.

{F8145658}

{F8145661}

Reviewers: #vdg, #plasma, jgrulich, ngraham

Reviewed By: #vdg, jgrulich, ngraham

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27792