matplotlib problem?

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

tweets_df = pd.read_csv('valid_tweets.csv')
#print(tweets_df)
print(pd.__version__)
pd.date_range('10/20/2016 1:55', '10/20/2016 3:55',
              freq='5 min', tz='UTC')

FIVEMIN = pd.offsets.Minute(5)

fig, ax = plt.subplots(figsize=(6, 3.5))

ax = (
    pd.read_csv('valid_tweets.csv', parse_dates=['Time'])
        .resample(FIVEMIN, on='Time')
        .count()
        .plot.line(ax=ax)
)