Beginner help on input and plotting

server.R

library(shiny)
#tu<-read.csv(file="tu.csv", header = TRUE, sep = ",")
#attach(tu)
tu <- data.frame(id=rnorm(1000),size=rnorm(1000),months=rnorm(1000))

shinyServer(function(input, output) {
  it<- reactive({
		print(input$tu)
		switch(input$tu,
			   "id" = tu$id,
			   "size" = tu$size,
			   "months" = tu$months)
		
	})
	
	output$distplot<- renderPlot({ hist(it())
								   
	})
})

ui.R

shinyUI(pageWithSidebar(
  
	
	headerPanel("statistics"),
	sidebarPanel( 
		selectInput("tu", "Variable:", list("id"="id","size"="size","months"="months")))
	, mainPanel(
		plotOutput("distplot")
		
	)
	
))