title: xlsxwriter charting issue when using time as the x-axis
body: <p>Taking a quick example:</p>
  
  <pre><code>Col A        Col B
  01:24:12     10
  05:23:12     11
  </code></pre>
  
  <p>What I'd like to happen is the x-axis should use the times as the category label but instead it always defaults to 00:00:00 so in the chart the x-axis is all 00:00:00.</p>
  
  <p>To setup the x-axis I use:</p>
  
  <pre><code>ctChart.set_x_axis({'name': 'Time', 'name_font': {'size': 14, 'bold': True}, 'num_font': {'italic': True}, 'date_axis': True, 'num_format': 'hh:mm:ss'})
  </code></pre>
  
  <p>Then to insert data:</p>
  
  <pre><code>timeFormat = workbook.add_format({'num_format': 'hh:mm:ss', 'align': 'left'})
  row = 1
  for t in ctTime:
      ctWorksheet.write_datetime(row, 0, t, timeFormat)               
      row += 1
  ctChart.add_series({'name': 'Power Data','categories': ['Power Data',1,0,len(ctTime),0], 'values': ['Power Data',1,1,len(ctList),1], 'line': {'color': 'red'}})
  </code></pre>
  
  <p>Interestingly, if I remove the 'num_format' from the set_x_axis command then it changes from 00:00:00 to 1,2,3,4 etc.  Seemingly the add_series command isn't recognozing that I want to use time indexing for the x-axis.  When in Excel and using the created workbook if I select a sample of A1:B2 and just hit 'line graph' it works perfectly.</p>