Using radio button selections for a plot in Shiny

Question

I'm attempting to make a widget on Shiny where two axes are selected to make a plot. My first thought was to make a data frame and radio buttons that select columns out of it. It's not working so far, and I'm not sure what is missing. What I'm starting with is:

library(shiny)
library(shinydashboard)

df <- data.frame(set_1=c(1,4,4,2,10,5),
      set_2=c(20,30,25,20,70,60),
      set_3=c(8,18,20,12,22,15))


db <- dashboardPage(
  dashboardHeader(title='My Dashboard'),
  dashboardSidebar(),
  dashboardBody(
    fluidRow(
      box(
        plotOutput('p1',height=250)
      ),
      
      box(
        title='Select x axis',
        radioButtons('radio_x',
                     choices=list('Set 1'='set_1',
                                  'Set 2'='set_2',
                                  'Set 3'='set_3'))
      ),
      
      box(
        title='Select y axis',
        radioButtons('radio_y',
                     choices=list('Set 1'='set_1',
                                  'Set 2'='set_2',
                                  'Set 3'='set_3'))
      )
    )
  )
)

server <- function(input,output){
  
  output$p1 <- renderPlot({
    p_data <- data.frame(x_axis=get(input$radio_x),y_axis=get(input$radio_y))
    plot(p_data$x_axis,p_data$y_axis)
  })
}

shinyApp(db,server)

The box where the plot is supposed to go says "Error: object 'list(1 = "1", 2 = "2", 3 = "3")' not found"

Any clues what needs to change here?