test.r

server <- function(input, output) {
  output$plot <- renderPlot({
    input$goPlot # Re-run when button is clicked
    
    dat <- data.frame(x = rnorm(10), y = rnorm(10))
    
    withProgress(message = 'Making plot', value = 1, {
      Sys.sleep(20)
    })
    
    plot(dat$x, dat$y)
  })
}

ui <- shinyUI(basicPage(
  tags$head(tags$style(HTML("
    .progress-striped .bar {
    background-color: #149bdf;
    background-image: -webkit-gradient(linear, 0 100%, 100% 0, color-stop(0.25, rgba(255, 255, 255, 0.6)), color-stop(0.25, transparent), color-stop(0.5, transparent), color-stop(0.5, rgba(255, 255, 255, 0.6)), color-stop(0.75, rgba(255, 255, 255, 0.6)), color-stop(0.75, transparent), to(transparent));
    background-image: -webkit-linear-gradient(45deg, rgba(255, 255, 255, 0.6) 25%, transparent 25%, transparent 50%, rgba(255, 255, 255, 0.6) 50%, rgba(255, 255, 255, 0.6) 75%, transparent 75%, transparent);
    background-image: -moz-linear-gradient(45deg, rgba(255, 255, 255, 0.6) 25%, transparent 25%, transparent 50%, rgba(255, 255, 255, 0.6) 50%, rgba(255, 255, 255, 0.6) 75%, transparent 75%, transparent);
    background-image: -o-linear-gradient(45deg, rgba(255, 255, 255, 0.6) 25%, transparent 25%, transparent 50%, rgba(255, 255, 255, 0.6) 50%, rgba(255, 255, 255, 0.6) 75%, transparent 75%, transparent);
    background-image: linear-gradient(45deg, rgba(255, 255, 255, 0.6) 25%, transparent 25%, transparent 50%, rgba(255, 255, 255, 0.6) 50%, rgba(255, 255, 255, 0.6) 75%, transparent 75%, transparent);
    -webkit-background-size: 40px 40px;
    -moz-background-size: 40px 40px;
    -o-background-size: 40px 40px;
    background-size: 40px 40px;
    }
    "))),
  plotOutput('plot', width = "300px", height = "300px"),
  actionButton('goPlot', 'Go plot')
  ))

shinyApp(ui = ui, server = server)