def __init__(self, pt, idx=None):
        """
        *idx* is a required attribute of a c:pt element, but must be
        specified when pt is None, as when a "placeholder" category is
        created to represent a missing c:pt element.
        """
        self._element = self._pt = pt
        self._idx = idx
def __init__ ( self , pt , idx = None ) : self . _element = self . _pt = pt self . _idx = idx
*idx* is a required attribute of a c:pt element, but must be
        specified when pt is None, as when a "placeholder" category is
        created to represent a missing c:pt element.