Added a small testcase which verifies that the history vector has an entry
with the current value after the first refresh after the initial access.
The end result looks like this:
https://phabricator.kde.org/file/data/mwuxhmkgb7p5gr2jl6ax/PHID-FILE-v76g3drarovozb4fue3w/KSysguard_with_per-process_cpu_graph