[Applet]Fix dimensions of traffic Monitor

Summary:
Remove top margin.
Workaround to get paintedheight for label. (Undefined or paintedheight
does not work.)
Base plotter's top margin on labels size.
Add small spacing at the bottom.
Add 0 value.
Align labels to the right.

Test Plan:
Before
{F8102889, inline=true, size=full}
After
{F8102890, inline=true, size=full}

Reviewers: #plasma, #vdg, jgrulich, ngraham

Reviewed By: #vdg, jgrulich, ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D27391