library(shiny)
library(datasets)

# We tweak the "am" field to have nicer factor labels. Since
# this doesn't rely on any user inputs we can do this once at
# startup and then use the value throughout the lifetime of the
# application
mpgData <- mtcars
mpgData$am <- factor(mpgData$am, labels = c("Automatic", "Manual"))


# Define server logic required to plot various variables against
# mpg
function(input, output) {

  # Compute the formula text in a reactive expression since it is
  # shared by the output$caption and output$mpgPlot functions
  formulaText <- reactive({
    paste("mpg ~", input$variable)
  })

  # Return the formula text for printing as a caption
  output$caption <- renderText({
    formulaText()
  })

  # Generate a plot of the requested variable against mpg and
  # only include outliers if requested
  output$mpgPlot <- renderPlot({
    boxplot(as.formula(formulaText()),
            data = mpgData,
            outline = input$outliers)
  })
}