title: How to download several png plots within one download button in shiny
body: <p>I am very new to shiny. I have several ggplot graghs. I added download button for each of them.<br>
  Here is an sample code:</p>
  
  <pre><code>output$salary_graph &lt;- renderPlot({
  print(salary_plot())
  })
  output$salary_plot_dl &lt;- downloadHandler(
    filename = function() {
      "salary_plot.png"
  },
  content = function(file) {
  png(file)
  print(salary_plot())
  dev.off()
  }
  )
  </code></pre>
  
  <p>I also have year_plot, group_plot and age_plot.</p>
  
  <p>Currently, I would like to add a button which can downloads all my png plots. It can be a zip file which contains 4 png files or pdf file with 4 pages or four separate png files.</p>
  
  <p>My questions here is not about creating a pdf or zip file to export all my plots in regular R script. I am asking the downloadHandler in the SHINY application. It is a unique question on this website.</p>
  
  <p>Can someone teach me how to do it? </p>