Set yaxis limits

        Parameters
        ----------
        low : number
        high : number
        index : int, optional

        Returns
        -------
        Chart

def ylim(self, low, high):
        """Set yaxis limits

        Parameters
        ----------
        low : number
        high : number
        index : int, optional

        Returns
        -------
        Chart

        """
        self.chart['yAxis'][0]['min'] = low
        self.chart['yAxis'][0]['max'] = high
        return self