ggplot object gets updated without formal assignment

[stitam]

Below is a simple example in which I prepare two scatter plots using the mtcars data set. var contains the name of the variable in mtcars which should be plotted on the y axis. Notice when I change the value of var the plot is automatically updated without any formal assignment. Does this happen on your end as well? Is this normal behaviour? Many thanks.

library(ggplot2)
data(mtcars)
var <- "cyl"
g <- ggplot(mtcars, aes(get(var), mpg)) + geom_point()
g

# change the value of var
var <- "wt"
# look at the original (!) plot
g

^{Created on 2024-07-11 with reprex v2.1.1}

This was prepared in a clean environment, R v4.4.1, ggplot2 v3.5.1

[teunbrand]

These are the consequence of delayed evaluation of the get() function in aes(). The plot object g doesn't get updated, the variable is looked up by get() when the plot is build (which happens during printing) rather than when plot components are declared.

For programatically giving variables by name, we instead recommend the .data[[var]] construct. This is documented in this section of the vignette. Please note that the second plot now hasn't changed.

library(ggplot2)

var <- "cyl"
g <- ggplot(mtcars, aes(.data[[var]], mpg)) + geom_point()
g

var <- "wt"
g

^{Created on 2024-07-11 with reprex v2.1.0}

[stitam]

Solves the issue, thanks @teunbrand.