The Sankey diagram is a graphical representation of quantity flows. The direction of movement of e.g. energy, material or money is indicated by arrows of different thickness, where the thickness of the arrows is proportional to the quantity. The first known graph was published by the French civil engineer Charles Joseph Minard in 1869, showing the losses of the French army during Napoleon’s Russian campaign. In 1898, the Irish engineer Captain Matthew Sankey visualized the energy flows of steam engines, which were published in the journal of the VDI in 1908.