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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def stack_type(self, stack_type):
 """Sets the stack_type of this ChartSettings. Type of stacked chart (applicable only if chart type is stacked). zero (default) means stacked from y=0. expand means Normalized from 0 to 1. wiggle means Minimize weighted changes. silhouette means to Center the Stream # noqa: E501 :param stack_type: The stack_type of this ChartSettings. # noqa: E501 :type: str """

        allowed_values = ["zero", "expand", "wiggle", "silhouette"]  # noqa: E501
        if stack_type not in allowed_values:
            raise ValueError(
                "Invalid value for `stack_type` ({0}), must be one of {1}"  # noqa: E501
                .format(stack_type, allowed_values)
            )

        self._stack_type = stack_type