title: xlsxwriter: How to change scatter chart subtype for single add_series()?
body: <p>I define a scatter chart using subtype smooth and add some data:</p>
  
  <pre><code>chart1 = workbook.add_chart({'type': 'scatter', 'subtype' : 'smooth'})
  Sname = "=Measurement_dBV!%s1" %(row_list[i])
  chart.add_series({
     'name':         Sname,
     'categories':   ['Measurement_dBV',1,0,numData,0],
     'values':       ['Measurement_dBV',1,i+1,numData,i+1],
     'line': {'width': 1.0, 'color': color_list[i]},
  })
  </code></pre>
  
  <p>This works fine. But now I need to add two other data lines to the same chart but in this case the lines must be a subtype of straight.
  My first idea was to use "combine". But the documentation say, that a chart of type scatter can´t be the primary chart as well as the combined chart is the same type.</p>
  
  <p>So: How to change the subtype for a single data series?</p>
  
  <p>In Excel you can define different types of charts an it´s subtypes for every single data series.</p>
  
  <p>Thanks for any help or suggestions.</p>
  
  <p>Best Regards
  Karsten </p>