Using switch to transform a variable from within a ggplot2 function - r

Question

I have an R Shiny app which plots horsepower from the mtcars data set against a user selected x variable. I would like the user to be able to select a transformation to perform on the x-variable ahead of plotting. In the simplified example below, these transformations are the ability to square it or obtain its reciprocal. I am doing this using a switch function.
While this switching function works in non-shiny contexts, I can't get it to execute inside a working shiny app. I know that I could perform the transformation on a reactive copy of the data frame and plot from that, but if possible, I would like to perform the transformation in the ggplot call itself. Does anyone have any suggestions on how to do so?
library(shiny)
library(tidyverse)
tran_func <- function(pred, trans) {
switch(trans,
"None" = pred,
"Reciprocal" = 1/pred,
"Squared" = pred^2,
)
}
ui <- fluidPage(
selectInput("xvar",
"Select X Variable",
choices = names(mtcars),
selected = "disp"),
selectInput("transform",
"Select Transformation",
choices = c("None", "Reciprocal", "Squared"),
selected = "None"),
plotOutput("scatter_good"),
plotOutput("scatter_bad")
)
server <- function(input, output, session) {
output$scatter_good <- renderPlot({
mtcars %>%
ggplot(aes_string(x = "hp", y = input$xvar)) +
geom_point()
})
output$scatter_bad <- renderPlot({
mtcars %>%
ggplot(aes_string(x = "hp", y = tran_func(input$xvar, "Squared"))) +
geom_point()
})
}
shinyApp(ui, server)

Answer 1

The issue would be the evaluation of the string passed from input$xvar to modify the column. An option is to pass the 'data' also as argument in the function, and use [[ to subset the column without converting to symbol or evaluate
library(shiny)
library(ggplot2)
library(dplyr)
tran_func <- function(data, pred, trans) {
switch(trans,
"None" = data[[pred]],
"Reciprocal" = 1/data[[pred]],
"Squared" = data[[pred]]^2,
)
}
ui <- fluidPage(
selectInput("xvar",
"Select X Variable",
choices = names(mtcars),
selected = "disp"),
selectInput("transform",
"Select Transformation",
choices = c("None", "Reciprocal", "Squared"),
selected = "None"),
plotOutput("scatter_good"),
plotOutput("scatter_bad")
)
server <- function(input, output, session) {
output$scatter_good <- renderPlot({
mtcars %>%
mutate(y_col = tran_func(cur_data(), input$xvar, input$transform)) %>%
ggplot(aes(x = hp, y = y_col)) +
geom_point()
})
output$scatter_bad <- renderPlot({
mtcars %>%
mutate(y_col = tran_func(cur_data(), input$xvar, input$transform)) %>%
ggplot(aes(x = hp, y =y_col)) +
geom_point()
})
}
shinyApp(ui, server)
-output