="desc" /></colgroup>
    <tr>
      <th>x</th>
      <td><p>A vector of class <code>Date</code>, <code>POSIXct</code> or <code>POSIXlt</code>,
of which the values are unique.</p></td>
    </tr>
    <tr>
      <th>start_format</th>
      <td><p>String to format the start values of each period, to be used
in <code>strftime</code>.</p></td>
    </tr>
    <tr>
      <th>end_format</th>
      <td><p>String to format the end values of each period, to be used
in <code>strftime</code>.</p></td>
    </tr>
    <tr>
