title: multi-level x axis in plotly
body: <p>I didn't find any information if multiple level x axis are available for plotly. I guess it is possible to do it in ggplot2 (eg. <a href="https://stackoverflow.com/questions/18165863/multirow-axis-labels-with-nested-grouping-variables">this</a> SO thread) and then transform it with <code>ggplotly()</code> but I would prefer a plotly solution. Is this possible? At the moment I am using an auxiliary column which concatenates year and month to the format "2017-12".</p>
  
  <p>Example dataset:</p>
  
  <pre><code>data.frame(
          year = c(2017, 2018, 2018, 2017, 2018, 2018),
         month = c(12, 1, 2, 12, 1, 2),
         value = c(120, 110, 130, 90, 100, 110)
  )
  </code></pre>
  
  <p>The result should look like this (year and month on the x axis):</p>
  
  <p><a href="https://i.stack.imgur.com/F0nYZ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F0nYZ.jpg" alt="enter image description here"></a></p>