minor edits to the very awesome app seen here: https://gist.github.com/jcheng5/3971908

server.R

# server.R

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

tg <- ToothGrowth
tg$dose <- factor(tg$dose)

shinyServer(function(input, output) {
     
    # Return the formula text for printing as a caption
    output$Caption <- renderText({
        paste(input$title)
    })

    # Dynamic plot of selected variable against mpg
    output$Plot <- renderPlot({
    
    # Figure out the aesthetic mappings
    # color_var requires special handling because it can be "none"
    if (input$color_var == "") {
      aes_mapping <- aes_string(x = input$x_var, y = "len")
    } else {
      aes_mapping <- aes_string(x = input$x_var, y = "len", fill = input$color_var)
    }
    
    # Make basic ggplot 
    p <- ggplot(tg, mapping = aes_mapping)
    
    # Add violin plot layer
    if (input$geom_violin) {
      p <- p + geom_violin(
        trim = input$violin_trim
        , adjust = input$violin_adjust
        , position = position_dodge(input$dodgewidth)
      )
    }

    # Add box plot layer
    if (input$geom_boxplot) {
      if (input$boxplot_outliers) {
        outlier_color <- "black"
      } else {
        outlier_color <- NA
      } 
      p <- p + geom_boxplot(
        width = input$boxplot_width
        , notch = input$boxplot_notch
        , outlier.colour = outlier_color
        , outlier.size = input$boxplot_outlier_size
        , position = position_dodge(input$dodgewidth)
      )
    }
    
    # Add dot plot layer
    if (input$geom_dotplot) {
      p <- p + geom_dotplot(
        binaxis = "y"
        , stackdir = input$dotplot_stackdir
        , method = input$dotplot_method
        , binwidth = input$dotplot_binwidth
        , alpha = input$dotplot_alpha
        , position = "dodge"
      )
    }
    
    # Add geom point layer
    if (input$geom_point) {
      p <- p + geom_point(
        shape = 21
        , size = input$point_size
        , colour = "black"
        , alpha = input$point_alpha
        , position = position_dodge(input$dodgewidth)
      )
    }
    
    # return the plot with selected layers
    print(p)
    
  }) # close output$outputPlot
  
}) # close shinyServer(function{

ui.R

# ui.R

library("shiny")

shinyUI(
  pageWithSidebar(
    headerPanel("Tooth Growth")
    ,
    sidebarPanel(
      textInput( 
        inputId = "title"
        , label = "Enter a title here:"
        , value = "plot"
      )
      ,
      selectInput( 
        inputId = "x_var"
        , label = "x variable:"
        , choices = c("Dose" = "dose", "Supplement Type" = "supp") 
      )
      ,
      selectInput(
        inputId = "color_var"
        , label = "fill color variable:"
        , choices = c("None" = "", "Dose" = "dose", "Supplement Type" = "supp")
        , selected = "Supplement Type"
      )
      ,
      conditionalPanel(
        condition = "input.color_var != 'none'"
        , tags$div( class = "well"
          , sliderInput(
              inputId = "dodgewidth"
              , label = "Horizontal dodging width (for different colors):"
              , min = 0.1
              , max = 1
              , value = .9
              , step = 0.1
            )
          )
      )
      ,
      checkboxInput(inputId = "geom_point", label = "Geom: point", value = FALSE)
      ,
      conditionalPanel(
        condition = "input.geom_point == true"
        , sliderInput(
            inputId = "point_alpha"
            , label = "Alpha (transparency):"
            , min = 0.0
            , max = 1
            , value = 1
            , step = 0.1
          )
        , sliderInput(
            inputId = "point_size"
            , label = "Size:"
            , min = 0.5
            , max = 8
            , value = 3
            , step = 0.5
          )
      )
      ,
      checkboxInput(inputId = "geom_dotplot", label = "Geom: dotplot", value = TRUE)
      ,
      conditionalPanel(
        condition = "input.geom_dotplot == true"
        , radioButtons(
          inputId = "dotplot_method"
          , label = "Binning method"
          , choices = c("Dot-density" = "dotdensity", "Histodot (regular spacing)" = "histodot")
        )
        ,
        sliderInput(
          inputId = "dotplot_binwidth"
          , label = "Bin width:"
          , min = 0.5
          , max = 3
          , value = 1
          , step = 0.25
        )
        ,
        sliderInput(
          inputId = "dotplot_alpha"
          , label = "Alpha (transparency):"
          , min = 0.0
          , max = 1
          , value = 1
          , step = 0.1
        )
        ,
        selectInput(
          inputId = "dotplot_stackdir"
          , label = "Stacking direction:"
          , choices = c("Centered" = "center", "Centered-aligned" = "centerwhole")
        )
      )
      ,
      checkboxInput(inputId = "geom_boxplot", label = "Geom: boxplot", value = FALSE)
      ,
      conditionalPanel(
        condition = "input.geom_boxplot == true"
        , sliderInput(
            inputId = "boxplot_width"
            , label = "Width:"
            , min = 0.1
            , max = 1
            , value = .5
            , step = 0.1
          )
        ,
        checkboxInput(inputId = "boxplot_notch", label = "Notched", value = FALSE)
        ,
        checkboxInput(inputId = "boxplot_outliers", label = "Show outliers", value = TRUE)
        ,
        conditionalPanel(
          condition = "input.boxplot_outliers == true"
          , sliderInput(
              inputId = "boxplot_outlier_size"
              , label = "Outlier size:"
              , min = 1
              , max = 8
              , value = 3
              , step = 1
            )
        )
      )
      ,
      checkboxInput(inputId = "geom_violin", label = "Geom: violin", value = FALSE)
      ,
      conditionalPanel(
        condition = "input.geom_violin == true"
        , sliderInput(
            inputId = "violin_adjust"
            , label = "Bandwidth adjustment ratio:"
            , min = 0.25
            , max = 2
            , value = 1
            , step = 0.25
          )
        , checkboxInput(inputId = "violin_trim", label = "Trim violins to data range", value = TRUE)
      )

    ) # close sidebarPanel(
    ,
    mainPanel(
      h3(textOutput("Caption"))
      ,
      plotOutput("Plot")
    ) # close mainPanel(
    
  ) # close pageWithSidebar(
) # close shinyUI(

Learning stuff from an app by Winston Chang & Joe Cheng,

https://gist.github.com/wch/3969102

https://gist.github.com/jcheng5/3971908