library(shiny)

ui <- 

  fluidPage(

    selectInput(

      inputId = "xvar",

      label = "Pick a variable for the x-axis:",

      choices = c("wt", "hp"),

      selected = "wt"

    ),

    plotOutput(outputId = "myPlot")

  )

server <- 

  function(input, output) {

    output$myPlot <- 

      renderPlot({

        plot(mtcars[, input$xvar], mtcars$mpg)

      })

  }

shinyApp(ui = ui, server = server, options = list(display.mode = "showcase"))