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This data is initially prepared and provided by the Bureau of the Fiscal Service in the form of a daily treasury statement, representing the cash operations of the federal government. Treasury.io reformats these statements, giving them a consistent structure, and provides the data as an open source, queryable database to the public. Treasury.io is not affiliated with the U.S. government.
More specifically, the chart above represents the withdrawals line items from "TABLE II - Deposits and Withdrawals of Operating Cash" of the Daily Treasury Statement. Whenever you select a new date, this page makes an API call to Treasury.io.
Note that daily treasury statements aren't issued every single day.