title: How to insert two X axis in a Matlab a plot
body: <p>I would like create a Matlab figure with a double X axis (m/s and km/h) with the same plot.</p>
  
  <p>I have found plotyy and - in Matlab reposity - plotyyy, but I am looking for:</p>
  
  <ol>
  <li>A double X axis.</li>
  <li>Together below the plot.</li>
  </ol>
  
  <p>My code is very simple:</p>
  
  <pre><code>stem(M(:, 1) .* 3.6, M(:, 3));
  
  grid on
  
  xlabel('Speed (km/h)');
  ylabel('Samples');
  </code></pre>
  
  <p><code>M(:, 1)</code> is the speed (in m/s), and <code>M(:, 3)</code> is the data.</p>
  
  <p>I would like only a second line, in the bottom, with the speeds in m/s.</p>