Decreases the zoom of the graph one step(0.1 units).

def ScaleSmaller(self):
    """Decreases the zoom of the graph one step(0.1 units)."""
    newfactor = self._zoomfactor - 0.1
    if float(newfactor) > 0 and float(newfactor) < self._MAX_ZOOM:
      self._zoomfactor = newfactor