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.
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.