Note: I see a $0 for gpt-4.1 mini on that day you show the calls, at the bottom of the graph in the legend where there are values shown. You getting billed less than the API calls you are making.
Are you enrolled in “Share inputs and outputs with OpenAI” for training, to get daily complementary tokens on chat models? That could be why you have the opposite of what you report - essentially no bill for gpt-4.1-mini on 2026-02-06.
If you want your own retrieval of costs, in buckets by day, you can create an ADMIN key, to use the organization admin endpoints.
Here’s my own “getting started” script for you, I’ve shared both help for “costs” and “usage” retrieval (essentially helping other API users answer similar mysteries):