*2.3. The Method for Evaluating Structural Changes of Sankey Diagrams–TRO index*

Through our literature review, we found that the method for evaluating structural changes of the Sankey diagram itself was still limited. It is difficult to systematically and quantitatively compare two Sankey diagrams and to discern the structural changes, because there are too many complex departments with detailed data in the diagram and the changes are reflected in many aspects. In order to solve this problem, we proposed TRO index to evaluate the structural changes, defining the structural change from three dimensions: total amount change, relative growth rate and occupation ratio change. In this study, specifically, the method was applied to compare carbon flow Sankey diagrams.

The meaning of TRO index is to help us quickly identify structural changes in complex carbon flow Sankey diagrams, including both obvious total amount changes and relative growth rate that is not easy to visualize. TRO is not only a mathematical indicator, but also has actual physical meaning, e.g., the total amount change reveals change in the industrial production capacity, the relative growth rate reveals potential development trends, and the occupation ratio change reflects the results of structural transition. These three indicators are complementary to each other for comprehensively revealing the structural changes. The meaning and calculation method of each index is explained in detail in the following text.
