A Stream graph is really close from a stacked area chart. It displays the evolution of a numerical value (Y axis) in function of another numerical value (X axis). This evolution is represented for several
groups, all with a distinct color. Contrary to a stacked area, there is no corner: edges are rounded what gives this nice impression of flow. Stream chart get really useful when displayed in an interactive mode:
highlighting a group gives you directly an insight of its evolution. R allows to easily make stream charts thanks to the streamchart library.
The stream graph library