code: def fetch_more(self, rows=False, columns=False):
          
          if  self.axis == 1 and self.total_rows > self.rows_loaded:
              reminder = self.total_rows - self.rows_loaded
              items_to_fetch = min(reminder, ROWS_TO_LOAD)
              self.beginInsertRows(QModelIndex(), self.rows_loaded,
                                   self.rows_loaded + items_to_fetch - 1)
              self.rows_loaded += items_to_fetch
              self.endInsertRows()
          if self.axis == 0 and self.total_cols > self.cols_loaded:
              reminder = self.total_cols - self.cols_loaded
              items_to_fetch = min(reminder, COLS_TO_LOAD)
              self.beginInsertColumns(QModelIndex(), self.cols_loaded,
                                      self.cols_loaded + items_to_fetch - 1)
              self.cols_loaded += items_to_fetch
              self.endInsertColumns()
docs: Get more columns or rows (based on axis).