The |ValueAxis| object providing access to properties of the value
        axis of this chart. Raises |ValueError| if the chart has no value
        axis.

def value_axis(self):
        """
        The |ValueAxis| object providing access to properties of the value
        axis of this chart. Raises |ValueError| if the chart has no value
        axis.
        """
        valAx_lst = self._chartSpace.valAx_lst
        if not valAx_lst:
            raise ValueError('chart has no value axis')

        idx = 1 if len(valAx_lst) > 1 else 0
        return ValueAxis(valAx_lst[idx])