title: Categorical values on the x-axis with xlsxwriter
body: <p>I have a plot that needs names as the values for the x-axis. I imagine this is accomplished via the set_x_axis function for chart objects but can't find the proper key in the documentation (<a href="http://xlsxwriter.readthedocs.io/chart.html#set_x_axis" rel="nofollow noreferrer">http://xlsxwriter.readthedocs.io/chart.html#set_x_axis</a>). The following code produces the graph below:</p>
  
  <pre><code>chart = workbook.add_chart({'type':'scatter'})
  colLetter = alphabet[1] #alphabet is list of alphabet
  for ii in range(4):
      colLetter = alphabet[ii+1]
      chart.add_series({
          'name': '=Sheet1!$%s$1'%colLetter,
          'categories': '=Sheet1!$A$2:$A$'+str(lastRowNumber),
          'values': '=Sheet1!$%s$2:$%s$6'%(colLetter, colLetter),
      })
  chart.set_title({'name': 'Cityblock'})
  chart.set_x_axis({'name': 'Subject'})
  chart.set_y_axis({'name': 'Distance'})
  chart.set_style(11)
  worksheet.insert_chart('F1', chart)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/lH2om.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lH2om.png" alt="enter image description here"></a></p>
  
  <p>Any suggestions? I'm using xlsxwriter with python 2.7.</p>