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caxlsx
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. Check out the README for some examples of how easy it is. Best of all, you can validate your xlsx file before serialization so you know for sure that anything generated is going to load on your client's machine.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Axlsx has been abandoned by the original author, but it still works for our needs. Our code that writes Excel workbooks hasn't changed in years. However, there is a community version called caxlsx that you might want to use instead.
For reading Excel files, we use roo. Our needs are more focused on simply extracting data from the files, so we don't need anything that can modify the files directly.
There is another library, rubyXL that I've been interested to try. It purports to read Excel files, but can save changes, making it a roundtrip capable library. This is something we can't currently do with our separate axlsx/roo workflow. We treat the Excel files as a kind of serializer input/output. The data all lives within our app.