I have written an introduction to this visualization. Zoom to any county by clicking on it. Clicking on the same county will zoom out. The scales preserve intensity for change over time: in other words, a color represents the same thing for each year on the map. However, the color scales do not necessarily preserve intensity from data field to data field: the darkest color for the total population does not represent the same values as for the enslaved population. The scales for population are logarithmic (with intermediate values) so every second step in the color ramp represents a ten-fold (not a two-fold) increase.
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.
The U.S. Census data and shapefiles for these maps comes from Minnesota Population Center, National Historical Geographic Information System, version 2.0 (Minneapolis, MN: University of Minnesota, 2011). For a description of the questions asked on the 1790 to 1860 censuses, see Measuring America: The Decennial Censuses From 1790 to 2000 (U.S Census Bureau, 2002). Bear in mind the reason the Census kept statistics on slavery. Slaves were counted in the Census because of the three-fifths compromise in the federal constitution, by which an enslaved person counted as three-fifths of a person when apportioning representation in Congress and direct taxes. I have tried to represent unavailable data on the map, but sometimes in the Census a value of zero actually means that the data has been lost or was never gathered. Treat the Census numbers skeptically: even in the best of circumstances the Census undercounts the population. For example, Harvey Amani Whitfield has shown that Vermont did have slavery, even though no slaves were enumerated in the Census. The numbers are useful chiefly for showing degrees of magnitude. Below are the fields in the NHGIS data that I have used. The total free population was always calculated by subtracting the slave population from the total population.