I believe it just did. I cannot thank you enough. Deadline for my thesis is coming up soon, and I really needed this to work out. Thanks man.
On a different note, I encountered the same bug as this guy
Where you tried to help him as well. I followed your advice and checked the heroku log, and it appeared that xlsxwriter was not imported; simply had to add it to requirements.txt.
Once again, thanks so much. Have a great one!
EDIT: the data for the excel is taken from charts, which vary on user inputs. Everytime I change an input, the excel is re-downloaded automatically. Do you know how to prevent this specific callback from being called every input change, and only called when I click on the button?