Tanimura, Susumu, Chusi Kuroiwa, and Tsutomu Mizota. “Proportional Symbol Mapping in R”. Journal of Statistical Software 15, no. 5 (January 24, 2006): 1–7. Accessed April 5, 2025. https://www.jstatsoft.org/index.php/jss/article/view/v015i05.