Fixing a crash caused by QCustomPlot's replot method in a concurrent thread

Summary:
Fixing an intermittent crash caused by QCustomPlot's replot method
being used in a concurrent thread The reason for it is because Qt does not
support painting to GUI elements in any thread but the GUI thread and
QCustomPlot's replot method includes a repaint. So if the Histogram is
currently visible and construct histogram is run in a concurrent thread,
the program would call replot and crash.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D17228