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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.
I could swear I found such a library a while back, but I can't find it now for love nor money. I did find an immutable hashmap, but it's written in MIT Scheme, which is a different dialect to Chez's R6RS. You could probably port it without a whole heap of effort, but it's not download-and-go I'm afraid. Sadly this is often the case in the Scheme world. I wish there was a larger community.
Racket is built on top of Chez Scheme and often has both mutable and immutable versions of data structures. I wonder if you can copy what you need from the source code.
Purely Functional Data Structures https://github.com/ijp/pfds