title: select time series data by month
body: <p>I have time series data that appears like below:</p>
  
  <pre><code>data
  
            Jan      Feb      Mar      Apr      May      Jun      Jul      Aug      Sep      
  1960 -10.3000  -7.1000   2.9000  18.0000  18.9000  29.5000  31.4000  28.0000  20.7000  
  1961  -1.8000  -4.2000   4.0000  13.2000  22.0000  23.4000  24.0000  23.4000  17.3000  
  1962 -12.1000  -3.1000  -0.1000   9.5000  22.9000  24.4000  26.4000  30.0000  21.2000  
  1963  -2.9000  -5.9000   5.4000   9.3000  21.0000  25.3000  27.9000  24.6000  21.4000  
  1964  -2.8000  -7.4000   2.2000   9.1000  17.6000  25.3000  25.1000  25.3000  18.0000  
  1965  -9.5000   1.0000   6.3000  14.3000  18.3000  24.7000  28.4000  26.7000  21.9000  
  1966  -5.4000  -2.7000   6.6000  14.3000  17.7000  24.2000  26.2000  25.3000  20.5000   
  1967  -8.1000  -0.6000   3.0000  13.8000  23.2000  23.8000  27.6000  24.3000  20.4000     
  </code></pre>
  
  <p>I want to plot the data on monthly scale, for example, I want to plot data of January month from 1960-1967. I tried using <code>zoo</code> function but it does not work. The problem is that even after installing <code>zoo</code> package, the <code>zoo</code> function seems to be unrecognized by the system. The error message looks like this:</p>
  
  <pre><code>&gt; install.packages("zoo")
  Installing package(s) into ‘C:/Users/skhanal2.RUSSELL/Documents/R/win-library/2.15’
  (as ‘lib’ is unspecified)
  trying URL 'http://streaming.stat.iastate.edu/CRAN/bin/windows/contrib/2.15/zoo_1.7-9.zip'
  Content type 'application/zip' length 868277 bytes (847 Kb)
  opened URL
  downloaded 847 Kb
  package ‘zoo’ successfully unpacked and MD5 sums checked
  Warning: cannot remove prior installation of package ‘zoo’
  
  The downloaded binary packages are in
          C:\Users\skhanal2.RUSSELL\AppData\Local\Temp\RtmpCSHWVD\downloaded_packages
  &gt; test&lt;-zoo(data,by="month")
  **Error: could not find function "zoo"**
  </code></pre>
  
  <p>Has anyone tried to plot time series data by month?If so, I would appreciate if someone could help me find the solution. Also, can anyone suggest what is the problem in <code>zoo</code> function? I am using R version 2.15.2 and I am using Windows 7 in 32 bit machine.  Thanks!</p>