图表上缺少Python 3上matplotlib的日期xticks。错误？

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“开始日期”轴上显示了xtick，而我没有。我的图表仅显示开始日期，没有提供日期。我试图将对象转换为datetime，但是它显示了日期并中断了它下面的图形，并且该行丢失了：

图

# Set as_index=False to keep the 0,1,2,... index. Then we'll take the mean of the polls on that day. 
poll_df = poll_df.groupby(['Start Date'],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 Difference 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')

笔记本是here