The graph shows the sad decline in the number of persons [men and women, despite my heading] wearing (or at least paying duty, at £1 3s 6d, on) hair powder between 1820 and 1832. It is from “Returns furnished by the Comptroller of Accounts in the Tax Office, expressly for this Work”, the work being Tables of the Revenue, Population, Commerce, &c of The United Kingdom and its dependencies Part III from 1820 to 1833, both inclusive. Compiled from official returns; presented to both Houses of Parliament by Command of His Majesty HMSO, London 1834, compiled by G R Porter, Board of Trade, Whitehall, in July 1834.
The duty was imposed under the Duty on Hair Powder Act 1795; the act was not repealed until 1869, by which time hardly anyone was paying it. Here is a very entertaining account of the whole business.
The inland waterways connection here is via the schooner Nancy, which took part in the Anglo-American war of 1812 on the Great Lakes between Canada and the United States of America. Stan Rogers included a song about the Nancy on his album From Fresh Water and you can hear him singing it here.
The song’s narrator did not like men in powdered hair.