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Hornik, Kurt. 2018. “R FAQ.” https://CRAN.R-project.org/doc/FAQ/R-FAQ.html.

Murrell, Paul. 2003. “Integrating Grid Graphics Output with Base Graphics Output.” R News 3 (2): 7–12.

Murrell, Paul, and Ross Ihaka. 2000. “An Approach to Providing Mathematical Annotation in Plots.” Journal of Computational and Graphical Statistics 9 (3): 582–99.