Toggling user-selected layers with ggplot2 in a Shiny app

server.R

library("shiny")
library("ggplot2")

shinyServer(function(input, output) {

  output$myplot <- renderPlot({

    ## Create a boxplot with optional jittering
    p <- ggplot(iris, aes(x = Species, y = Petal.Length)) +
      geom_boxplot() +
      ylab("Petal Length")
    if (input$optlayer) {
      p <- p + geom_jitter()
    }
    p
  })
})

ui.R

library("shiny")

shinyUI(pageWithSidebar(

  ## App title
  titlePanel("Toggling User-Selected Layers"),

  # Sidebar with a layer that user can choose to add
  sidebarPanel(
    checkboxInput("optlayer", label = "Jitter", value = FALSE)
   ),
    
    ## Show plot in main panel
    mainPanel(
      plotOutput("myplot")
    )
  )
)

Lovely, nice solution to the problem :-)