Fig. 8

Analysis of keyword co-occurrence. A 245 keywords were divided into 5 clusters by different colors (cluster I: red, cluster II: green, cluster III: blue, cluster IV: yellow, cluster V: purple) with the minimum co-occurrence times of 5. The size of the node represents the co-occurrence frequency and the closeness of the co-occurrence relationship with other keywords. B Visual network map of keywords according to the APY. The more yellow the node is, the later the keyword occurrences. C Density visualization map for 245 keywords. Each keyword in the density map shows the frequency of its occurrence in related research fields. Red represents the high-frequency keyword, and the position of the keyword is related to the degree of correlation between it and other keywords. The position in the center indicates the stronger correlation with the others