Adjust the scrollbar position to take into account the zooming of
        the figure.

def _adjust_scrollbar(self, f):
        """
        Adjust the scrollbar position to take into account the zooming of
        the figure.
        """
        # Adjust horizontal scrollbar :
        hb = self.horizontalScrollBar()
        hb.setValue(int(f * hb.value() + ((f - 1) * hb.pageStep()/2)))

        # Adjust the vertical scrollbar :
        vb = self.verticalScrollBar()
        vb.setValue(int(f * vb.value() + ((f - 1) * vb.pageStep()/2)))