Reacts to mouse wheel movement, custom behaviour switches zoom
        axis when ctrl is pressed, and sets the locus of zoom, if zeroWheel
        is set.

def wheelEvent(self, ev, axis=None):
        """Reacts to mouse wheel movement, custom behaviour switches zoom
        axis when ctrl is pressed, and sets the locus of zoom, if zeroWheel
        is set."""
        state = None
        # ctrl reverses mouse operation axis
        if ev.modifiers() == QtCore.Qt.ControlModifier:
            state = self.mouseEnabled()
            self.setMouseEnabled(not state[0], not state[1])
        if self._zeroWheel:
            ev.pos = lambda : self.mapViewToScene(QtCore.QPoint(0,0))
        super(SpikeyViewBox, self).wheelEvent(ev, axis)
        if state is not None:
            self.setMouseEnabled(*state)