var active = null;

function displayTip(id, e) {
  if(active != null && active != id)
    clearTip();

  active = id;

  var left = e.clientX + document.documentElement.scrollLeft;
  var top = e.clientY + document.documentElement.scrollTop + 20;
  var whichTip = document.getElementById(active);
  whichTip.style.left = left + 'px';
  whichTip.style.top = top + 'px';
  whichTip.style.visibility = "visible";
}

function clearTip() {
  var whichTip = document.getElementById(active);
  whichTip.style.visibility = "hidden";
  active = null;
}