def plot_trajectory(proj_file, dir_file, show=False):
    """ Plot optimization trajectory on the plane spanned by given directions."""

    assert exists(proj_file), 'Projection file does not exist.'
    f = h5py.File(proj_file, 'r')
    fig = plt.figure()
    plt.plot(f['proj_xcoord'], f['proj_ycoord'], marker='.')
    plt.tick_params('y', labelsize='x-large')
    plt.tick_params('x', labelsize='x-large')
    f.close()

    if exists(dir_file):
        f2 = h5py.File(dir_file,'r')
        if 'explained_variance_ratio_' in f2.keys():
            ratio_x = f2['explained_variance_ratio_'][0]
            ratio_y = f2['explained_variance_ratio_'][1]
            plt.xlabel('1st PC: %.2f %%' % (ratio_x*100), fontsize='xx-large')
            plt.ylabel('2nd PC: %.2f %%' % (ratio_y*100), fontsize='xx-large')
        f2.close()

    fig.savefig(proj_file + '.pdf', dpi=300, bbox_inches='tight', format='pdf')
    if show: plt.show()