RTLSDR Panorama VSWR plot

import os, sys, string, math
import pygal

def getFileData2(i_fName):
    lines = open(i_fName).readlines()
    res= {}
    for i in xrange(len(lines)):
        l = lines[i]
        tokens = string.split(l)
        freq = int(tokens[2][:-1]) / 1000000
        power = float(tokens[-1])
        res[freq] = power
    return res

def listDiff(a, b):
    Len  = min(len(a), len(b))
    res = [None] * Len
    for i in xrange(Len):
        res[i] = a[i] - b[i]
    return res

def ReduceArray(i_arr, maxCount = 30):
    skip = len(i_arr)/(maxCount-1)
    if skip==0:
        skip = 1
    return i_arr[0::skip]

def PlotLines2d(x_data, y_data, outFile):
    line_chart = pygal.Line(style=pygal.style.DarkSolarizedStyle, logarithmic=True, x_label_rotation=90, height=500, width=1600)
    line_chart.title = ''

    line_chart.x_labels = map(str, ReduceArray(x_data[:], 100))

    line_chart.add('VSWR', ReduceArray(y_data, 100), show_dots=False)
    line_chart.render_to_file(outFile)

def main():
    pNoAnt = getFileData2(sys.argv[1]) # no antenna connected
    pWithAnt = getFileData2(sys.argv[2]) # antenna connected
    freqs = list(set(pNoAnt.keys()) & set(pWithAnt.keys()))
    freqs = sorted(freqs)

    # leave only samples present in both files
    for k in pNoAnt.keys():
        if k not in freqs:
            del pNoAnt[k]

    for k in pWithAnt.keys():
        if k not in freqs:
            del pWithAnt[k]

    returnLoss = [None] * len(freqs)
    for i in xrange(len(freqs)):
        returnLoss[i] = max( .0001, pNoAnt[freqs[i]] - pWithAnt[freqs[i]] )
    print returnLoss

    vswr = map(lambda x: (pow(10, x/20) + 1)/(pow(10, x/20) - 1), returnLoss)
    print vswr
    PlotLines2d(freqs, vswr, sys.argv[3])


if __name__ == '__main__':
    if len(sys.argv) < 4:
        print '''Usage:\n \
VSWR.py noAntennaConnected.csv antenaConnected.csv outputPlot.svg
        '''
    else:
        main()