code>TRUE</code></p></td>
    </tr>
    <tr>
      <th>na.rm</th>
      <td><p>logical indicating whether NA's should be removed.</p></td>
    </tr>
    <tr>
      <th>na.value</th>
      <td><p>numeric indicating where NA's should be substituted to trigger the NA color.</p></td>
    </tr>
    <tr>
      <th>show.grid</th>
      <td><p><code>TRUE</code> to show gridlines, <code>FALSE</code> to hide them, 
or a numeric value to specify the gridline thickness in pixels (can 
be a non-integer).</p></td>
    </