Sankey Diagrams display flows and their quantities in proportion to one another. The width of the arrows or lines are used to show their magnitudes, so the bigger the arrow, the larger the quantity of flow.
Flow arrows or lines can combine together or split through their paths on each stage of a process. Colour can be used to divide the diagram into different categories or to show the transition from one state of the process to another.
Typically, Sankey Diagrams are used to visually show the transfer of energy, money or materials, but they can be used to show the flow of any isolated system process.