header.html

<script type="text/javascript">

// toggle visibility of R source blocks in R Markdown output
function toggle_R() {
  var x = document.getElementsByClassName('r');
  if (x.length == 0) return;
  function toggle_vis(o) {
    var d = o.style.display;
    o.style.display = (d == 'block' || d == '') ? 'none':'block';
  }

  for (i = 0; i < x.length; i++) {
    var y = x[i];
    if (y.tagName.toLowerCase() === 'pre') toggle_vis(y);
  }

    var elem = document.getElementById("myButton1");
    if (elem.value === "Show/Hide Code") elem.value = "Show/Hide Code";
    else elem.value = "Show/Hide Code";
}

document.write('<input onclick="toggle_R();" type="button" value="Show/Hide Code" id="myButton1" style="position: absolute; top: 10%; right: 2%; z-index: 200"></input>')

document.addEventListener('DOMContentLoaded', toggle_R);

</script>