A map of the spread of slavery in the United States, 1790 to 1860, based on U.S. Census data from NHGIS.
Map by Lincoln Mullen. Code licensed MIT. Built with D3.js.
If you use this map in your research, I would appreciate a citation. Here is the suggested form:
Lincoln Mullen, "The Spread of U.S. Slavery, 1790--1860," interactive map, http://lincolnmullen.com/projects/slavery/, doi: 10.5281/zenodo.9825.
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You should also cite the NHGIS:
Minnesota Population Center, National Historical Geographic Information System: Version 2.0 (Minneapolis, MN: University of Minnesota, 2011), http://www.nhgis.org.