title: Return pmin or pmax of data.frame with multiple columns
body: <p>Is there a way to select the pmax/pmin of a data frame with multiple columns??</p>
  
  <p>I want only the max or min returned, not the entire row.</p>
  
  <pre><code>max &lt;- tail(df, n=1)
  max
  #                       v1     v2     v3     v4     v5     v6     v7     v8
  #2014-10-03 17:35:00  58.91  45.81  33.06  70.76  36.39  45.53  33.52  34.36
  
  pmax(max)
  #                       v1     v2     v3     v4     v5     v6     v7     v8
  #2014-10-03 17:35:00  58.91  45.81  33.06  70.76  36.39  45.53  33.52  34.36
  </code></pre>
  
  <p>For this row, I expect a return value of :</p>
  
  <pre><code>70.76
  </code></pre>
  
  <p>...as it is the maximum value across all the columns.</p>