A Sankey diagram is a flow diagram that visualizes the movement of quantities between different stages or categories. The width of each flow is proportional to the quantity it represents. These diagrams are especially useful for showing how inputs are distributed into outputs and where losses occur along a process.
A classic example is in energy flow analysis. A Sankey diagram can show how primary energy sources like coal, gas, or renewables are converted and distributed to different end uses, such as industry, households, or transport. It clearly highlights how much energy is lost during conversion or transmission.
Other use cases include customer journey mapping or budget allocation.