Save the plot.

        Returns
        -------
        str
            The basename with which the plot has been saved.

def _save_plot(self, *args, extension='pdf', **kwargs):
        """Save the plot.

        Returns
        -------
        str
            The basename with which the plot has been saved.
        """
        import matplotlib.pyplot as plt

        tmp_path = make_temp_dir()
        filename = '{}.{}'.format(str(uuid.uuid4()), extension.strip('.'))
        filepath = posixpath.join(tmp_path, filename)

        plt.savefig(filepath, *args, **kwargs)
        return filepath