Sets the dataset instance associated with this item.
        
        :param      dataset | <XChartDataset>

def setDataset(self, dataset):
        """
        Sets the dataset instance associated with this item.
        
        :param      dataset | <XChartDataset>
        """
        self._dataset = dataset
        
        # setup the tooltip
        tip = []
        tip.append('<b>%s</b>' % dataset.name())
        for value in dataset.values():
            value_text = []
            for key, val in sorted(value.items()):
                if val == dataset.name():
                    continue
                
                axis = self.axis(key)
                if axis and axis.labelFormat():
                    val = axis.labelFormat().format(val)
                value_text.append('%s: %s' % (key, val))
            
            tip.append('<p>%s</p>' % ', '.join(value_text))
        self.setToolTip(''.join(tip))