title: rolling mean with python 2.7
body: <p>I want to write a rolling mean  code of m_tax using Python 2.7 pandas to analysis the time series data from the web page (<a href="http://owww.met.hu/eghajlat/eghajlati_adatsorok/bp/Navig/202_EN.htm" rel="nofollow">http://owww.met.hu/eghajlat/eghajlati_adatsorok/bp/Navig/202_EN.htm</a>). </p>
  
  <pre><code>   datum  m_ta m_tax     m_taxd m_tan     m_tand
  ------- ----- ----- ---------- ----- ----------
  1901-01  -4.7   5.0 1901-01-23 -12.2 1901-01-10
  1901-02  -2.1   3.5 1901-02-06  -7.9 1901-02-15
  1901-03   5.8  13.5 1901-03-20   0.6 1901-03-01
  1901-04  11.6  18.2 1901-04-10   7.4 1901-04-23
  1901-05  16.8  22.5 1901-05-31  12.2 1901-05-05
  1901-06  21.0  24.8 1901-06-03  14.6 1901-06-17
  1901-07  22.4  27.4 1901-07-30  16.9 1901-07-04
  1901-08  20.7  25.9 1901-08-01  14.7 1901-08-29
  ....
  </code></pre>
  
  <p>Here I tried my code as:</p>
  
  <pre><code> pd.rolling_mean(df.resample("1M", fill_method="ffill"), window=60,   min_periods=1, center=True).mean()
  </code></pre>
  
  <p>and I got result:</p>
  
  <pre><code>m_ta            11.029173
  m_tax           17.104283
  m_tan            4.848637
  month            6.499500
  monthly_mean    11.030405
  monthly_std      1.836159
  m_tax%           0.083348
  m_tan%           0.023627
  dtype: float64
  </code></pre>
  
  <p>In another way I tried as:</p>
  
  <pre><code>s = pd.Series(np.random.randn(1000), index=pd.date_range('1/1/1900',   periods=1000))
  s = s.cumsum()
  r = s.rolling(window=60)
  r.mean()
  </code></pre>
  
  <p>and I got result</p>
  
  <pre><code>1900-01-01          NaN
  1900-01-02          NaN
  1900-01-03          NaN
  1900-01-04          NaN
  1900-01-05          NaN
  1900-01-06          NaN
  1900-01-07          NaN
  1900-01-08          NaN
  ...
  </code></pre>
  
  <p>So I am confused here. Which one should I use?  Could someone please give me idea? Thanks!</p>