Manually specify the x-axis tick labels.

        :param row_column_list: a list containing (row, column) tuples to
            specify the subplots, or None to indicate *all* subplots.
        :type row_column_list: list or None
        :param labels: list of tick labels.

def set_xtick_labels_for_all(self, row_column_list=None, labels=None):
        """Manually specify the x-axis tick labels.

        :param row_column_list: a list containing (row, column) tuples to
            specify the subplots, or None to indicate *all* subplots.
        :type row_column_list: list or None
        :param labels: list of tick labels.

        """
        if row_column_list is None:
            for subplot in self.subplots:
                self.set_xtick_labels(subplot.row, subplot.column, labels)
        else:
            for row, column in row_column_list:
                self.set_xtick_labels(row, column, labels)