def hide_zeroes(self):
        '''
        Sometimes it makes sense to hide the labels for subsets whose size is zero.
        This utility method does this.
        '''
        for v in self.subset_labels:
            if v is not None and v.get_text() == '0':
                v.set_visible(False)
Sometimes it makes sense to hide the labels for subsets whose size is zero.
        This utility method does this.