title: Pandas: save dataframe to open Workbook
body: <p>I've got an open workbook which data is written to. How can I add sheets to write the content of a dataframe?</p>
  
  <pre><code>import xlsxwriter
  import pandas as pd
  
  workbook = xlsxwriter.Workbook('test.xlsx')
  sheet1 = workbook.add_worksheet(name='sheet1')
  sheet1.write(0, 0, 'test')
  df = pd.DataFrame({'a': [1, 2, 3, 4], 'b': ['aa', 'bb', 'cc', 'dd']})
  # write df somehow to next sheet. I usually use df.to_excel(filename, engine='xlsxwriter'), but this will create a new file
  workbook.close()
  </code></pre>