Returns a list of the data sets that are assigned with this
        chart widget.
        
        :param      visible | <bool>
        
        :return     [<XChartDataSet>, ..]

def datasets(self, visible=True):
        """
        Returns a list of the data sets that are assigned with this
        chart widget.
        
        :param      visible | <bool>
        
        :return     [<XChartDataSet>, ..]
        """
        if visible is not None:
            return filter(lambda x: x.isVisible(), self._datasets)
        return self._datasets[:]