'set the units for axis
ACCEPTS: a units tag'

def set_units(self, u):

    pchanged = False
    if (u is None):
        self.units = None
        pchanged = True
    elif (u != self.units):
        self.units = u
        pchanged = True
    if pchanged:
        self._update_axisinfo()
        self.callbacks.process(u'units')
        self.callbacks.process(u'units    finalize')
    self.stale = True
