'Return the tzinfo instance of *x* or of its first element, or None'

@staticmethod
def default_units(x, axis):

    if isinstance(x, np.ndarray):
        x = x.ravel()
    try:
        x = cbook.safe_first_element(x)
    except (TypeError, StopIteration):
        pass
    try:
        return x.tzinfo
    except AttributeError:
        pass
    return None
