The source data are the revised International Sunspot Numbers (ISN v2.0), as available at SILSO.
[1] Sunspot counts exist since 1610[2] but the cycle numbering is not well defined during the Maunder minimum.
[3] It was proposed that one cycle might have been lost in the late 18th century,[4] but this remains not fully confirmed.
Other smoothing formulas exist, and they usually give slightly different values for the amplitude and timings of the solar cycles.
An example is the Meeus smoothing formula,[7] with related solar cycles characteristics available in this STCE news item.