Saving a iplot and it shows blank image

Question

The code

    plt.figure(figsize=(12,6))
    df2 = pd.read_csv('C:\Users\madanraj.c\Downloads\qeport.csv',encoding = "ISO-8859-1")
    df2['Date']  = df2['Last Resolved Date'].apply(lambda t: pd.to_datetime(t).date())
    df2['DateStr']  = df2['Date'].apply(lambda t: str(t))
    df4 = pd.DataFrame(df2['Assignee+'].value_counts())
    df4.iplot()
    fig=plt.gcf()
    fig.savefig('on.png')


#ax.fig.savefig('output.png')

os.getcwd()

The code give the output as expected, but it doesnt save the image.