Fig. 6.

(a) Co-occurrence. Each color represents a thematic cluster, bubble size indicated occurrence of the keyword, and thickness of the line revealed linkage strength between keywords. (b) Overlay visualization of thematic terms. The bubble colors indicated the average publication date of keywords. The purple color indicated the former publication date. The yellow color indicated later publication date. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)