import matplotlib
matplotlib.use("Qt4Agg")
import matplotlib.pyplot as plt
...

import matplotlib
matplotlib.use("agg")
import matplotlib.pyplot as plt
...

...
import os, sys, argparse

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--noX", action="store_true", dest="noX")
    params = parser.parse_args()

    data = [0,1,2,3,4,5]
    fig, ax = plt.subplots()
    ax.plot(data)
    if not params.noX:
        plt.show()
    plt.savefig("foo.png")

    sys.exit(0)

if __name__ == "__main__":
    main()