Alluvial diagrams use x-splines, sometimes augmented with stacked histograms, to visualize multi-dimensional or repeated-measures data with categorical or ordinal variables. They can be viewed as simplified and standardized Sankey diagrams; see Riehmann, Hanfler, and Froehlich (2005) [doi:10.1109/INFVIS.2005.1532152](https://doi.org/10.1109/INFVIS.2005.1532152) and Rosvall and Bergstrom (2010) [doi:10.1371/journal.pone.0008694](https://doi.org/10.1371/journal.pone.0008694). This package provides ggplot2 layers to produce alluvial diagrams from tidy data.