title: Pass string in add_series in xlsxwriter
body: <p>I have created graph using add series method.is it possible to pass string into add_series method as I am creating worksheet with string value ( depend on some condition).
  In below code, Can I replace Graph with string.
  name = "abhi"</p>
  
  <pre><code>chart.add_series({
      'categories' : '=name!$B$32:$B$532',
      'values': '=name!$C$32:$C$532',
      'name':'clk2q0_S',
      'line':{'color':'blue'}})
  </code></pre>