python时间轴问题

平均几分钟写入一次记录数据，想利用python做趋势，几年的数据，做出来如图：

X轴太多，加载在一起了，所以，想修改X轴坐标，结果报错：

Traceback (most recent call last):
  File "D:\share\d.py", line 26, in <module>
    plt.xticks(pd.date_range(new11.index[0],new11.index[-1],freq='M'),rotation=45)
  File "C:\Anaconda2\lib\site-packages\pandas\tseries\index.py", line 2051, in date_range
    closed=closed, **kwargs)
  File "C:\Anaconda2\lib\site-packages\pandas\util\decorators.py", line 91, in wrapper
    return func(*args, **kwargs)
  File "C:\Anaconda2\lib\site-packages\pandas\tseries\index.py", line 267, in __new__
    ambiguous=ambiguous)
  File "C:\Anaconda2\lib\site-packages\pandas\tseries\index.py", line 427, in _generate
    start = Times**p(start)
  File "pandas\tslib.pyx", line 295, in pandas.tslib.Times**p.__new__ (pandas\tslib.c:9203)
  File "pandas\tslib.pyx", line 1343, in pandas.tslib.convert_to_tsobject (pandas\tslib.c:25854)
TypeError: Cannot convert input to Times**p

代码如下：

1. #!/usr/local/miniconda2/envs/science/bin/python
   
2. #-*- coding: utf-8 -*-
   
3. import pandas as pd
   
4. import matplotlib.pyplot as plt
   
5. import matplotlib.dates as mdate
   
6. import MySQLdb
   
7. 
   
8. db=MySQLdb.connect(host="127.0.0.1",user='root',port=3306,db="test")
   
9. sql2016="SELECT * FROM (SELECT DATE_FORMAT(Timee,'%Y-%m-%d %H:%i') AS riqi,PH AS 2016ph FROM table1 ) t WHERE t.riqi = 2016"
   
10. sql2017="SELECT * FROM (SELECT DATE_FORMAT(Timee,'%Y-%m-%d %H:%i') AS riqi,PH AS 2017ph FROM table1 ) t WHERE t.riqi = 2017"
    
11. df2017=pd.read_sql(sql2017,con=db)
    
12. df2016=pd.read_sql(sql2016,con=db)
    
13. 
    
14. new=pd.concat([df2016,df2017],axis=1)
    
15. new_df=new[["riqi","2016ph","2017ph"]]
    
16. new11=new_df.set_index("riqi")
    
17. 
    
18. fig = plt.figure()
    
19. ax = fig.add_subplot(111)
    
20. ax.xaxis.set_major_formatter(mdate.DateFormatter('%Y-%m-%d'))
    
21. 
    
22. 
    
23. plt.xticks(pd.date_range(new11.index[0],new11.index[-1],freq='M'),rotation=45)
    
24. ax.plot(new11.index,new11['2016ph','2017ph'],color='r')
    
25. new11.plot()
    
26. plt.show()

复制代码

其中new11中值如下：

想问怎么修改，能实现，X轴以月为单位，谢谢！