Scale value inside of aes_string()

Question

I want to scale my y-var by multiplying it by a number, say 10, in ggplot. The problem is this is in a Shiny app and the variable must be passed as a character string, i.e. input$variable.

How can I multiply one of the variables in aes_string() the same way I would in aes()? Here is an example of when it fails:

 library(ggplot2)
 ggplot(data = subset(mtcars, cyl == 4), aes_string(x = "wt", y = "mpg")) + 
      geom_line(size = 1.5, color = "#00868B") + 
      geom_line(data = subset(mtcars, cyl == 8), aes_string(x = "wt", y = "mpg" * 10))

Error in "mpg" * 10 : non-numeric argument to binary operator

Answer 1

You can use tidyeval approach introduced in ggplot2 v3.0.0

# install.packages("ggplot2", dependencies = TRUE)
library(ggplot2)

var1 <- "wt"
var2 <- "mpg"
multiplier <- 10

ggplot(data = subset(mtcars, cyl == 4), 
       aes(x = !! rlang::sym(var1), y = !! rlang::sym(var2))) + 
  geom_line(size = 1.5, color = "#00868B") + 
  geom_line(data = subset(mtcars, cyl == 8), 
            aes(x = !! rlang::sym(var1), y = !! rlang::sym(var2) * multiplier))

Or put everything in a function

plot_select_vars <- function(var1, var2, multiplier) {

  var1 <- rlang::sym(var1)
  var2 <- rlang::sym(var2)

  ggplot(data = subset(mtcars, cyl == 4), 
         aes(x = !! var1, y = !! var2)) + 
    geom_line(size = 1.5, color = "#00868B") + 
    geom_line(data = subset(mtcars, cyl == 8), 
              aes(x = !! var1, y = !! var2 * multiplier))

}

plot_select_vars(var1, var2, multiplier)

Created on 2018-06-06 by the reprex package (v0.2.0).

Answer 2

I prefer to use get instead of aes_string to call variables inside ggplot2 and it works with value modification, for example:

library(ggplot2)
X <- "wt"
Y <- "mpg"
ggplot(subset(mtcars, cyl == 4), aes(get(X), get(Y))) + 
    geom_line() + 
    geom_line(data = subset(mtcars, cyl == 8), aes(y = get(Y) * 10)) +
    labs(x = X,
         y = Y)

PS: you don't need to call wt in second aes as it's the same "variable" as in first aes.