Figure 2.

Annual publication status (A), major productive countries distribution (B), and Country Collaboration Map (C). Each node represents a country, and the line between the two nodes indicates that they have a co-occurrence relationship. The larger the node means the greater the volume of national publications. The closer the distance, the stronger the relationship. Moreover, the more yellow the color tends to be, the more cutting-edge the research is (A,B). The red path signifies the partnership between the countries, and the wider the red means the closer the cooperation (C).