Added citation information

README.md

@@ -1,7 +1,28 @@
-A map of the spread of slavery in the United States, 1790 to 1860, by
-[Lincoln Mullen][]. See the map for citations to the data. Code licensed
-[MIT][]. Built with [D3.js][].
+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][].
+
+## Citation
+
+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>.
+
+  [map of the spread of slavery]: http://lincolnmullen.com/projects/slavery/
+  [NHGIS]: https://www.nhgis.org/
   [Lincoln Mullen]: http://lincolnmullen.com
   [MIT]: http://lmullen.mit-license.org/
   [D3.js]: http://d3js.org
+  []: https://zenodo.org/badge/3774/lmullen/slavery-map.png

index.html

@@ -51,36 +51,53 @@ <h5>How to use this map</h5>
     <p>
     I have written an <a 
       href="http://lincolnmullen.com/blog/the-spread-of-american-slavery/">introduction 
-    to this visualization</a>. 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.
+      to this visualization</a>. 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.
     </p>
     <hr>
     <p>
     Map by <a href="http://lincolnmullen.com">Lincoln Mullen</a>. Code 
     available on <a href="https://github.com/lmullen/slavery-map">
-    GitHub</a>, licensed MIT. Suggestions and criticism welcome: <a 
-    href="mailto:[email protected]">[email protected]</a>.
+      GitHub</a>, licensed MIT. Suggestions and criticism welcome: <a 
+      href="mailto:[email protected]">[email protected]</a>.
+    </p>
+    <h5>Citation</h5>
+    <p>
+    If you use this map in your research, I would appreciate a citation.
+    Here is the suggested form:
+    <blockquote>
+      Lincoln Mullen, "The Spread of U.S. Slavery, 1790&ndash;1860," interactive
+      map, http://lincolnmullen.com/projects/slavery/, doi:
+      10.5281/zenodo.9825.
+    </blockquote>
+
+    You should also cite the NHGIS:
+
+    <blockquote>
+      Minnesota Population Center, *National Historical Geographic Information 
+      System: Version 2.0* (Minneapolis, MN: University of Minnesota, 2011), http://www.nhgis.org.
+    </blockquote>
     </p>
     <hr>
     <h5>Explanation of Census data</h5>
     <p>
     The U.S. Census data and shapefiles for these maps comes from Minnesota 
     Population Center, <a href="https://www.nhgis.org/"><em>National 
-    Historical Geographic Information System</em></a>, version 2.0 
-(Minneapolis, MN: University of Minnesota, 
+        Historical Geographic Information System</em></a>, version 2.0 
+    (Minneapolis, MN: University of Minnesota, 
     2011). For a description of the questions asked on the 1790 to 1860 
-         censuses, see <a 
-    href="http://www.census.gov/population/www/censusdata/hiscendata.html"><em>Measuring 
-  America: The Decennial Censuses From 1790 to 2000</em></a> (U.S Census 
-  Bureau, 2002). Bear in mind the reason the Census kept statistics on 
-  slavery. Slaves were counted
+    censuses, see <a 
+      href="http://www.census.gov/population/www/censusdata/hiscendata.html"><em>Measuring 
+        America: The Decennial Censuses From 1790 to 2000</em></a> (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 <a 
       href="http://en.wikipedia.org/wiki/Three-Fifths_Compromise">three-fifths</a> 
     compromise in the federal constitution, by which an enslaved person 
@@ -89,8 +106,8 @@ <h5>Explanation of Census data</h5>
     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 <a 
-        href="http://www.uvm.edu/research/?Page=news&storyID=17688&category=uvmresearch">
+    population. For example, Harvey Amani Whitfield has shown that <a 
+      href="http://www.uvm.edu/research/?Page=news&storyID=17688&category=uvmresearch">
       Vermont did have slavery</a>, 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