How to conditionally choose which variables to include in a plot (ggplot2)?

Question

I would like to create a function that takes a column name and creates a plot based on that. For example, I want to be able to plot the Petal.Length column of the iris dataset against other variables. The way to do it without indirection is

ggplot(data = iris) + geom_point(x = Petal.Width, y = Petal.Length)

This is the plot I would like to replicate through indirection, but none of the following attempts work. These two return similar undesired plots:

colname = "Petal.Width"
ggplot(data = iris) + geom_point(x = colname, y = Petal.Length)

ggplot(data = iris) + geom_point(x = {{colname}}, y = Petal.Length)

The following attempt does not work either, it returns an error:

ggplot(data = iris) + geom_point(aes(x = !!!rlang::syms(colname), y = Petal.Length))

#> Warning in geom_point(aes(x = !!!rlang::syms(colname_x), y = Petal.Length)):
#> Ignoring unknown aesthetics:
#> Error in `geom_point()`:
#> ! Problem while setting up geom.
#> ℹ Error occurred in the 1st layer.
#> Caused by error in `compute_geom_1()`:
#> ! `geom_point()` requires the following missing aesthetics: x

Any hint on how we could do this? The idea is to have a function that is able to plot that kind of chart just by giving a string corresponding to one x variable that appears in the dataset.

Answer 1

Using the .data pronoun and by wrapping inside aes() you could do:

library(ggplot2)

colname <- "Petal.Width"
ggplot(data = iris) + 
  geom_point(aes(x = .data[[colname]], y = Petal.Length))

Answer 2

Your plot #2 is invariant in x because it takes "Petal.Width" as a literal value (as in data.frame(x="Petal.Width")), obviously not what you intend.

There are a few ways to work with programmatic variables:

1. We can use the .data pronoun in ggplot, as in

   var1 <- "mpg"
   var2 <- "disp"
   ggplot(mtcars, aes(x = .data[[var1]], y = .data[[var2]])) +
     geom_point()
   

2. We can use rlang::quo and !! for interactive work:

   x <- rlang::quo(mpg)
   y <- rlang::quo(disp)
   ggplot(mtcars, aes(x = !!x, y = !!y)) +
     geom_point()
   

3. If in a function, we can enquo (and !!) them:

   fun <- function(data, x, y) {
     x <- rlang::enquo(x)
     y <- rlang::enquo(y)
     ggplot(data, aes(!!x, !!y)) +
       geom_point()
   }
   fun(mtcars, mpg, disp)
   

Answer 3

another (more clumsy) approach with base R:

colname = "Petal.Width"
ggplot(data = iris) + 
  geom_point(aes(x = eval(parse(text = colname)), y = Petal.Length)
)