renderUI with GGPLOT

server.r

library(shiny) # load the shiny package
library(ggplot2) # load the gglpot2 package if ploting using ggplot

# beginning of the shiny server function
shinyServer(function(output, input)({
  
  # The following reactive function would return the column variable names corresponding to the dataset selected by the user.
  var <- reactive({
    switch(input$data1,
           "mtcars" = names(mtcars),
           "trees" = names(trees)
           )
  })
  
  # vx and vy are the output variable from renderUI containing the list of variable names in a drop down/selectInput UI widget
  # renderUI is used in server side and is used along with uiOutput in the ui.r
  # uiOuput is used in ui.R to display the selectInput widget dynamically using the output variable vx & vy
  # using vx and vy in the ui.R we will dynamically create the drop down with the column names based on the dataset selected by the user

  output$vx <- renderUI({
selectInput("variablex", "Select the First (X) variable", choices = var())
  })
  
  output$vy <- renderUI({
selectInput("variabley", "Select the Second (Y) variable", choices = var())
  })

# renderPlot is used to plot the ggplot and the plot output will be stored in the output variable p which could be used in ui.r to display the plot
  output$p <- renderPlot({

    # attach(get(input$data1))
    # plot(x= get(input$variablex), y= get(input$variabley), xlab=input$variablex, ylab=input$variabley)
    data = switch(input$data1,
           "mtcars" = mtcars,
           "trees" = trees
           )
           
           x= get(input$variablex)
           y= get(input$variabley)
    # If ggplot needs to de drawn the following could also be used instead of the plot () function
    # ggplot(data, aes_string(x=input$variablex, y=input$variabley), environment = environment()) + 
    # geom_point(aes(get(input$variablex), get(input$variabley))) 
    # OR alternatively the following could also be used
    # data = get(input$data1)
    # x = get(input$variablex)
    # y=get(input$variabley)
    
    ggplot(data=data, aes(x=x, y=y), environment = environment()) + geom_point(aes(x=x, y=y))

  })
  
}))

ui.r

library(shiny)

shinyUI(fluidPage(
  titlePanel("Dynamic user interface - RenderUI"),
  sidebarLayout(
    sidebarPanel(
      
      # select input with the list of datasets
      selectInput(inputId = "data1", label = "Select the Dataset of your choice", choices = c("mtcars","trees")),
      br(),
      helpText("The following selectInput drop down choices are dynamically populated based on the dataset selected by the user above"),
      br(),
      uiOutput("vx"), # vx is coming from renderUI in server.r
      br(),
      br(),
      uiOutput("vy") # vy is coming from renderUI in server.r

      ),
    mainPanel(
      plotOutput("p")
      )
    
    )
 ))