Who are the ggplot2 extenders, and how can you become one? An Overview

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Since its release, the R package ggplot2 (Wickham 2007) has become a widely used tool for data visualization. Based on the "grammar of graphics" (Wilkinson 1999), ggplot2 provides a flexible framework for creating highly customizable graphics. In addition, it was designed to be extendable; that is, to allow users to create their own grammar components (e.g. themes, stats, geoms, and scales). Consequently, since ggplot2's release nearly two decades ago, the number of extension packages has grown into the hundreds. Some add new functionality via the prescribed extension mechanisms while others introduce variations in dialect into the grammar, tailored to domain-specific needs. This paper attempts to characterize this ecosystem and highlight pathways for individuals to enter the extension space.