This may seem like a simple figure, but it actually represents a huge number of different variables. This is a sankey diagram of sorts, but there are some additional features. This figure represents the "metabolic pattern in Shanghai" as calculated by using a methodology the authors of the associated publication call the Multi-Scale Integrated Analysis of Societal and Ecosystem Metabolism (MuSIASEM) approach. In this approach, land use and other variables are used to analyze the metabolism of a city. Here is what is being represented in this graph (emphasis ours):

[This figure] presents the analysis results of the urban metabolic pattern in Shanghai for the year of 2013. The funds are represented by the circles with human figures. The size of each circle represents the amount of managed land allocated to this sector. The number of human figures represents the proportion of human time allocated to this sector. The described flows represent energy throughput and GDP output. The breadth of the flows represents the associated quantity.

That means there are four different variables being shown in a graph that has a very simple appearance. Kudos to the authors for fitting in this large quantity of information, while still keeping this graph very easy to read.