GNU R alluvial plots in 'ggplot2'

 Alluvial plots use variable-width ribbons and stacked bar plots to
 represent multi-dimensional or repeated-measures data with categorical or
 ordinal variables; see Riehmann, Hanfler, and Froehlich (2005)
 <doi:10.1109/INFVIS.2005.1532152> and Rosvall and Bergstrom (2010)
 <doi:10.1371/journal.pone.0008694>.
 Alluvial plots are statistical graphics in the sense of Wilkinson (2006)
 <doi:10.1007/0-387-28695-0>; they share elements with Sankey diagrams and
 parallel sets plots but are uniquely determined from the data and a small
 set of parameters. This package extends Wickham's (2010)
 <doi:10.1198/jcgs.2009.07098> layered grammar of graphics to generate
 alluvial plots from tidy data.