It was implemented by the Graph and Sequence GraphController derived
classes and was used
1) in FunctionGraphController::viewWillAppear to set the cursorView
specific to each derived class,
2) in FunctionGraphController::cursor/displayBottomMarginRatio()
methods.
Instead
1) the viewWillAppear method of each derived classes sets its
cursorView itself
2) one may already get the cursorView through curveView().
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