Manually specify the x-axis tick labels.

        :param row,column: specify the subplot.
        :param labels: list of tick labels.

def set_xtick_labels(self, row, column, labels):
        """Manually specify the x-axis tick labels.

        :param row,column: specify the subplot.
        :param labels: list of tick labels.

        """
        subplot = self.get_subplot_at(row, column)
        subplot.set_xtick_labels(labels)