ส่วนที่ขึ้น error JpGraph Error: HTTP headers have already been sent. Caused by output from file result.php at line

<?php
include("dbconn.inc.php");
if (!isset($_SESSION)) {
  session_start();
}
$topic_id = $_GET['tid'];
$qid = $_GET['qid'];
$sql = "SELECT title, num_votes FROM topic WHERE topic_id = $topic_id;";
$result = mysql_query($sql);

$num_votes = mysql_result($result, 0, "num_votes");
if($num_votes == 0) {
 	die("<h2>�ѧ����ռ����ǵ��Ǣ�͹��</h2>");
}
	$sql = "select * from guestion where ques_id = $qid order by ques_id;";
	$resultmain = mysql_query($sql);
	while ($datamain = mysql_fetch_array($resultmain)) { // loop table question
	$question = $datamain['ques_title'];
	 
		include("JpGraph/src/jpgraph.php");
		include("JpGraph/src/jpgraph_pie.php");
		$g = new PieGraph(500, 500);
		
		$title = "ผลลัพธ์";
		$g->title->Set($title);
		$g->title->SetFont(FF_JASMINE, FS_BOLD, 18);
		
		$topic = mysql_result($result, 0, "title");
		$subtitle = $question;
		$g->subtitle->Set($subtitle);
		$g->subtitle->SetFont(FF_CORDIA, FS_NORMAL, 16);
		
		$sql = "SELECT * FROM choice WHERE ques_id = $qid;";
		$result = mysql_query($sql);
		
		$legends = array();
		$data_graph = array();
		while($data = mysql_fetch_array($result)) {
			$lg = $data['item'];
			array_push($legends, $lg);
			array_push($data_graph, $data['score']);
		}
		
		$pie = new PiePlot($data_graph);
		$pie->SetLegends($legends);
		$g->legend->SetFont(FF_CORDIA, FS_NORMAL, 12);
		
		$g->Add($pie);
		$g->Stroke();

?>
<?  } ?>