function setfv() {
    f=document.getElementById('fv').value;
  }
    a = 2.5;
    xmode = 0;
    thestroke = "blue";
  function toggle() {
    if (xmode == 1) { thestroke = "blue"; xmode = 0; }
    else { thestroke = "red"; xmode = 1; }
    // alert("update");
    update();
    }