(Builds on: Exploratory data analysis (2D))
R uses NA
to present unknown, but potentially measurable variables.
The pattern of missingness in a dataset is often informative, so you should never silently ignore missing values.
Missing values [r4ds-5.2.3]. (transformation)
Missing values [r4ds-5.6.2]. (summary)
Missing values [r4ds-7.4]. (visualisation)