'Set padding of X data limits prior to autoscaling.
*m* times the data interval will be added to each
end of that interval before it is used in autoscaling.
accepts: float in range 0 to 1'

def set_xmargin(self, m):

    if ((m < 0) or (m > 1)):
        raise ValueError(u'margin    must    be    in    range    0    to    1')
    self._xmargin = m
    self.stale = True
