Python 3上matplotlib的图表上缺少xticks

Question

我正在关注本节，我意识到这段代码是使用Python 2编写的，但是它们在'Start Date'轴上显示了xticks，而我没有。我的图表仅显示Start Date，没有提供日期。

    ＃设置as_index = False保留0,1,2，...索引。然后，我们将以当天的平均调查结果为准。     poll_df = poll_df.groupby（['开始日期']，as_index = False）.mean（）

# Let's go ahead and see what this looks like
poll_df.head()
Start Date  Number of Observations  Obama   Romney  Undecided   Difference
0   2009-03-13  1403    44  44  12  0.00
1   2009-04-17  686 50  39  11  0.11
2   2009-05-14  1000    53  35  12  0.18
3   2009-06-12  638 48  40  12  0.08
4   2009-07-15  577 49  40  11  0.09
Great! Now plotting the Differencce versus time should be straight forward.

# Plotting the difference in polls between Obama and Romney
fig = poll_df.plot('Start Date','Difference',figsize=(12,4),marker='o',linestyle='-',color='purple')

https://nbviewer.jupyter.org/github/jmportilla/Udemy-notes/blob/master/Data%20Project%20-%20Election%20Analysis.ipynb