Creating a Chart on a separate tab prevents all worksheets from loading.
If you run the attached python script, the following is printed: ['Sheet1', 'Chart1', 'Sheet2']
However, the actual number of sheets in the workbook is: ['Sheet1', 'Chart1', 'Sheet2', 'Sheet3']
This happens because of:
sheet_codename = 'sheet%d.xml' % (i + 1)
in reader/excel.py doesn't consider that some codenames are actually considered chartsheets (e.g., 'xl/chartsheets/sheet1.xml' versus 'xl/worksheets/sheet1.xml')