在shiny提供的例子中，如何下载直方图？

library(shiny)# Define UI for application that plots random distributions shinyUI(pageWithSidebar(  # Application title  headerPanel("Hello Shiny!"),  # Sidebar with a slider input for number of observations  sidebarPanel(    sliderInput("obs",                 "Number of observations:",                 min = 1,                max = 1000,                 value = 500)  ),  # Show a plot of the generated distribution  mainPanel(    plotOutput("distPlot")  )))library(shiny)# Define server logic required to generate and plot a random distributionshinyServer(function(input, output) {  # Expression that generates a plot of the distribution. The expression  # is wrapped in a call to renderPlot to indicate that:  #  #  1) It is "reactive" and therefore should be automatically   #     re-executed when inputs change  #  2) Its output type is a plot   #  output$distPlot <- renderPlot({    # generate an rnorm distribution and plot it    dist <- rnorm(input$obs)    hist(dist)  })})