Shiny intro tutorial example rewritten for ggplot2

server.R

library(shiny)
library(datasets)
library(ggplot2) # load ggplot

# Define server logic required to plot various variables against mpg
shinyServer(function(input, output) {
  
  # Compute the forumla text in a reactive function since it is 
  # shared by the output$caption and output$mpgPlot functions
  formulaText <- reactive(function() {
    paste("mpg ~", input$variable)
  })
  
  # Return the formula text for printing as a caption
  output$caption <- reactiveText(function() {
    formulaText()
  })
  
  # Generate a plot of the requested variable against mpg and only 
  # include outliers if requested
  # ggplot version
  
   output$mpgPlot <- reactivePlot(function() {
     # check for the input variable
     if (input$variable == "am") {
       # am
       mpgData <- data.frame(mpg = mtcars$mpg, var = factor(mtcars[[input$variable]], labels = c("Automatic", "Manual")))
     }
     else {
       # cyl and gear
       mpgData <- data.frame(mpg = mtcars$mpg, var = factor(mtcars[[input$variable]]))
     }

     p <- ggplot(mpgData, aes(var, mpg)) + 
       geom_boxplot(outlier.size = ifelse(input$outliers, 2, NA)) + 
       xlab(input$variable)
     print(p)
   })
  
})