Set header data

def headerData(self, section, orientation, role=Qt.DisplayRole):
        """Set header data"""
        if role != Qt.DisplayRole:
            return None
        if orientation == Qt.Vertical:
            return six.text_type(self.color_da.cmap[section].values)
        return super(ColormapModel, self).headerData(section, orientation,
                                                     role)