Pythonで電圧・電流の時系列CSVからグラフを作成する

import pandas as pd
dat = pd.read_csv(fname, header=None)

#!/usr/bin/python3
import sys
import os

import matplotlib.pyplot as plt
import pandas as pd

def drawcsv(fname):
    if not os.path.exists(fname):
        raise Exception('File not found:' + fname)

    # CSVファイルの読み込み(ヘッダ行無しのデータ)
    dat = pd.read_csv(fname, header=None)

    # グラフのプロット
    fig, ax1 = plt.subplots()

    color = 'tab:blue'
    ax1.set_xlabel('Time(min.)')
    ax1.set_ylabel('Voltage(V)', color=color)
    ax1.plot(dat[0]/60, dat[1], color=color)
    ax1.tick_params(axis='y', labelcolor=color)

    ax2 = ax1.twinx()  # instantiate a second axes that shares the same x-axis

    color = 'tab:red'
    ax2.set_ylabel('Current(mA)', color=color)  # we already handled the x-label with ax1
    ax2.plot(dat[0]/60, dat[2], color=color)
    ax2.tick_params(axis='y', labelcolor=color)

    fig.tight_layout()  # otherwise the right y-label is slightly clipped
    ax1.set_ylim(10,12.5)
    plt.show()    

if __name__ == '__main__':
    if len(sys.argv)>1:
        drawcsv(sys.argv[1])

    ax1.set_ylim(10,12.5)