Axlsx Show archive.org snapshot is an incredible gem to generate "Office Open XML" spreadsheet files (XLSX). Does not break on large spreadsheets and supports a ton of features like graphs.

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API looks mature and existing code is easy to migrate when coming from the spreadsheet gem.
The documentation of some methods is a bit out of date, but you'll find your way around the gem's code.

No support for reading files, however. :( If you want to open XLSX spreadsheets (for example to confirm your output in tests), you can use roo Show archive.org snapshot .

To easily dump Rails records to XLSX, there is also axlsx_rails Show archive.org snapshot (haven't tried it yet).

xlsx generation with charts, images, automated column width, customizable styles and full schema validation.
Axlsx excels at helping you generate beautiful Office Open XML Spreadsheet documents without having to understand the entire ECMA specification.