Draw Parabola by JS_2

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>Draw Parabola by JS (2)</title>
</head>
<body>
<script>
        var _max = 500;
        var _counter = 0;
        var _step = 20;

        document.write("<svg class=rainbow xmlns=svg xmlns:xlink=http://www.w3.org/2000/svg width=1000 height=1000>");
        for ( _counter = 0 ; _counter < _max ; _counter = _counter + _step) {
        var _r =  _counter;
        var _g =  ( _counter * 10 ) % 255;
        var _b =  ( _counter * 100 ) % 255;

                document.write("<line x1=0 y1=" + _counter + " x2=" + _counter + " y2=" + _max + " ");
                document.write("style='stroke:rgb(" + _r + "," + _g + "," + _b + ");stroke-width:2;'></line>");
        }

        document.write("</svg>");
</script>
</body>
</html>