Method initializeParentPlot

! \internal Sets the parent plot of this layerable. Use this function once to set the parent plot if you have passed 0 in the constructor. It can not be used to move a layerable from one QCustomPlot to another one. Note that, unlike when passing a non-null parent plot in the constructor, this function does not make \a parentPlot the QObject-parent of this layerable. If you want thi