What's the best data structure to use to store this data?

That depends on your definition of "best", the amount of data, the number of queries, and the precision of your data and query data.

Two possibilities:

• An array, where the values are kept in order. To query, perform a binary search.
• A hash, with entries for every minute (or second, depending on the precision of your query data). Missing entries in the raw data are just copied.

Abigail