Beginner: Javascript must be on header to run? Does the declaration order of the functions matter?

<head>
      window.onload = function(){
         var x = new Array(0,2,3,4,5,6,7,8);
         var y = new Array(20,10,40,30,60,50,70,10);
         drawGraph(y,x);
      }
</head>

function addLoadEvent(func) {
    var oldonload = window.onload;
    if (typeof window.onload != 'function') {
      window.onload = func;
    }
    else {
      window.onload = function() {
        oldonload();

        var x = new Array(0,2,3,4,5,6,7,8);
        var y = new Array(20,10,40,30,60,50,70,10);
        drawGraph(y,x);
      }
    }
}

$(document).ready(function() {
    var x = new Array(0,2,3,4,5,6,7,8);
    var y = new Array(20,10,40,30,60,50,70,10);
    drawGraph(y,x);
  })

<html>
    <head>
        <script type="text/javascript">
            function check_existance()
            {
                if(!check_existance.i)
                    check_existance.i = 0;

                document.write("<h5>Call : " + ++check_existance.i + "</h5>" +
                      "func1 : " + typeof func1 + "<br />" +
                      "func2 : " + typeof func2 + "<br />" +
                      "func3 : " + typeof func3 + "<br />" +
                      "func4 : " + typeof func4 + "<br />");
            }
        </script>
        <script type="text/javascript">
            check_existance();

            func1 = function()
                    {
                        alert("func1");
                    };

            check_existance();

            function func2()
            {
                alert("func2");
            }
        </script>
    </head>
    <body>
        <script type="text/javascript">
            check_existance();

            func3 = function()
                    {
                        alert("func3");
                    };

            check_existance();

            function func4()
            {
                alert("func4");
            }
        </script>
    </body>
</html>