It looks like this is a community continuation of the axlsx gem which was maintained back in the day by Randy Morgan (randym) over at https://github.com/randym/axlsx. One of my earliest open source contributions was adding support so that you could output spreadsheets with "conditional" formatting (color something red if it is below some value, for instance). I remember Randy being extremely supportive of new contributors and it made me want to be a part of the ruby community.
Good memories of the open source world. I couldn't find my commits in either repo and i'm afraid i might have been shy to upstream them and just did them in a fork. it was tiny, There was some issue in some less number formatting variation involving currencies in multiple locales. we needed xlsx for its ability to do some nice formatting etc but i really wanted to have a need for things like generating charts, embedding scripts etc just for the sheer nerdiness
Thanks for continuing the work!