In a Sankey diagram, lines or arrows of variable widths represent quantities flowing from one stage (node) to another. The chart is a directed acyclic graph (DAG). After reaching a stage, a flow may split or join other flows that arrive from previous stages, but never return to a previously visited stage. In most Sankey charts nodes are placed on a vertical grid, uniformly spaced, while curves flow through them. Nodes are usually represented as rectangles or vertical lines and sometimes hidden, to reveal only the flows.
Charles J. Minard's dramatic representation of Napoleon's attempt to invade Russia in 1812, uses a Sankey chart to reveal the tragic the fate of the French soldiers while they marched from Kovno (present-day Kaunas, Lithuania) to Moscow and back. The nodes are placed on geographical locations and the chart is also linked to a line chart...
