Boxplot in python using pandas, matplotlib | Vary position of figure

boxplot.py

import pandas as pd
import matplotlib.pyplot as plt
import sys
import os
import matplotlib as mpl

pd.options.display.mpl_style = 'default'

def box_plot():
    f = open(sys.argv[1], "r")
    image_name = os.path.splitext(sys.argv[1])[0]+".pdf"

    vals = pd.DataFrame.from_csv(f, sep='\t', index_col=False) #, columns=['20'])

    fig = plt.figure();
    ax = fig.add_subplot(111)
    bp = vals.boxplot()
    plt.xlabel('Loss (%)')
    plt.ylabel('Packet receipt rate (%)')

    # sets font size
    font = {'family' : 'normal',
        'weight' : 'bold',
        'size'   : 18}

    mpl.rc('font', **font)
    # gets axes position
    fig_pos = ax.get_position()
    # sets axes position; done to move figure vertically
    ax.set_position([fig_pos.x0, fig_pos.y0+0.045, fig_pos.width, fig_pos.height])
    plt.savefig(image_name, format='pdf')

box_plot()