title: checkbox not selected in a shiny app
body: <p>I need to prepare a shiny app for a school project.
  This is a link of what it is supposed to look like</p>
  
  <p><a href="https://yuvaln.shinyapps.io/olympics/" rel="nofollow">https://yuvaln.shinyapps.io/olympics/</a></p>
  
  <p>If you look at the app you see there is a checkbox named medals.When you
  open the app they are all selected but in the event the user decides to uncheck them all there should be a small error and no graph should be drawn.</p>
  
  <p>I am having trouble getting to this, when I uncheck all the boxes in my app 
  it draws an empty drawing</p>
  
  <p>This is the important part of the code:</p>
  
  <pre><code> fluidRow(
    column(3,checkboxGroupInput("Medals", label = strong("Medals"),
    choices = list("Total" = "TOTAL", "Gold" = 'GOLD',
    "Silver" = 'SILVER','Bronze'='BRONZE'),
     selected = c('TOTAL','GOLD','SILVER','BRONZE')))),
  
    fluidRow(
      mainPanel(plotOutput('coolplot'),width = '40%'))
  )    
  )
    server &lt;- function(input, output){output$coolplot&lt;-renderPlot(plot.medals2(input$country,
    input$Startingyear,input$Endingyear,input$Medals))}
    shinyApp(ui = ui, server = server)
  </code></pre>
  
  <p>I am using a function plot.medals2 that gets a vector of medals ,start year, ending year, country and  returns a drawing of the graph.</p>