title: how to plot time series with x axis ticks in month-year format in ggplot
body: <p>How to plot the data below in <code>ggplot</code> with x axis to show ticks in format as 'Jan-99'.</p>
  
  <p>My data is as below:</p>
  
  <pre><code>head(rates,10)
  
      Month Repo_Rate
  1  Apr-01      9.00
  2  May-01      8.75
  3  Jun-01      8.50
  4  Jul-01      8.50
  5  Aug-01      8.50
  6  Sep-01      8.50
  7  Oct-01      8.50
  8  Nov-01      8.50
  9  Dec-01      8.50
  10 Jan-02      8.50
  
  sapply(rates,class)
  
  #  Month   Repo_Rate 
  # "character"   "numeric" 
  </code></pre>
  
  <p>I have done the plotting  using <code>xts</code>/<code>zoo</code>/<code>ts</code> packages but would like to do using <code>ggplot</code> as this gives my publication quality figures.</p>