Two plugins were also added to expand the basic functionality of Leaflet. The first is Esri-Leaflet, a plugin that can include ESRI services. This was used for adding the historic map layers as tiled Rest services. Each map has three options for base layers. The two historic map overlays are courtesy of the David Rumsey Map Collection: the 1790 Celestial Globe Gores by Giovanni Maria Cassini, the 1812 The World on Mercator's projection by John Pinkerton, and a modern map layer provided by ArcGIS.com.
The second plugin is Leaflet.markercluster, which makes expandable clusters for overlapping points. Single place marks represent one point on the map. Within the placemark is a number showing how many quotes are associated with it. After zooming in, the place mark spiders out into multiple points that can be selected individually. A popup box appears with the quote, place name, and source letter number.