As stated by the source, taxpayers could elect to assign $3 of the income tax reported on their returns ($6 on a joint return, if both spouses contributed) to help pay for the cost of Presidential election campaigns. To do so, a taxpayer had to check a box on the tax return. The relationship between (a) the number of returns and boxes checked and (b) the dollar amounts is only approximate because the frequencies and the amounts were obtained from different sources, the frequencies from Statistics of Income tax return data and the dollar amounts from IRS monthly tax collections, aggregated by calendar year. It should be noted that the frequencies and amounts are related to the tax year for which statistics are shown, even though the designation by the taxpayer occurred in the following year, when the income tax return was filed and processed.
For additional information about the return filing requirements, see the annual reports, Statistics of Income-Individual Income Tax Returns at https://www.irs.gov/uac/soi-tax-stats-historical-data-tables.
U.S. Department of the Treasury. Internal Revenue Service, Individual Income Tax Filing: Returns with Presidential Election Campaign Fund Check off [RTNBPECFC], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/RTNBPECFC, February 21, 2019.