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Data visualization methods are discussed in books in the data mining area, for example, Shmueli et al. [2017] and Williams [2011]. In this chapter, we mostly use the graphics functions from base R. A popular dedicated graphics package in R is the ggplot2 package by Wickham [2016]. The ggplot2 package provides more flexible and powerful graphics capability that can create presentation-quality visualization. However, it also comes with a significant learning curve to get familiar with the special technical language used in ggplot2. For those who use data visualizations on a regular basis, it is worth the time and effort to learn ggplot2.

Sample statistics such as sample mean vector and sample covariance matrix for multivariate observations are discussed in detail in many multivariate statistics books, for example, Johnson et al. [2002] and Rencher [2003].