Port TrafficMonitor from KQuickControlsAddons.Plotter to KQuickCharts

KDeclarative will be split up for KF6 and stuff removed. This is the only place
using Plotter so it will likely be not kept around.
I tried to keep behavior visuals the same but could not recreate the hardcoded
gradients of Plotter. One change that is intentional is reducing the line
thickness from 2px to 1px because it looks much better in my opinion.