title: How to find top n% of records in a column of a dataframe using R
body: <p>I have a dataset showing the exchange rate of the Australian Dollar versus the US dollar once a day over a period of about 20 years.  I have the data in a data frame, with the first column being the date, and the second column being the exchange rate.   Here's a sample from the data:</p>
  
  <pre><code>&gt;data
               V1     V2
  1    12/12/1983 0.9175
  2    13/12/1983 0.9010
  3    14/12/1983 0.9000
  4    15/12/1983 0.8978
  5    16/12/1983 0.8928
  6    19/12/1983 0.8770
  7    20/12/1983 0.8795
  8    21/12/1983 0.8905
  9    22/12/1983 0.9005
  10   23/12/1983 0.9005
  </code></pre>
  
  <p>How would I go about displaying the top n% of these records?  E.g. say I want to see the days and exchange rates for those days where the exchange rate falls in the top 5% of all exchange rates in the dataset?</p>