title: XlsxWriter error for percent format
body: <p>I'm using Pandas and exporting data to excel using XlsxWriter. 
  One of the data columns has floats and needs to be formatted as percent, so this is  how I do it: </p>
  
  <pre><code>percent_fmt = workbook.add_format({'num_format': '0.00%'})
  worksheet.set_column('E:E', percent_fmt)
  </code></pre>
  
  <p>After that the following error appears:</p>
  
  <blockquote>
    <p>File "C:\Program
    Files\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 4688,
    in _write_col_info
        / float(max_digit_width) * 256.0) / 256.0</p>
    
    <p>TypeError: unsupported operand type(s) for *: 'Format' and 'int'</p>
  </blockquote>
  
  <p>What am I doing wrong here?</p>