急：图片上的中文显示乱码怎么办？

Question

?>
<?php
//柱状图生成类
//修正：http://www.codefans.net
//function createImage($data,$twidth,$tspace,$height){
                        $dataName = array();
                        $dataValue = array();
                        $i = 0;
                        $j = 0;
                        $k = 0;
                        $num = sizeof($data);                        foreach($data as $key => $val){
                                        $dataName[] = $key;
                                        $dataValue[] = $val;
                                }
        
                        $maxnum = max($data);
                        $width = ($twidth + $tspace) * $num + 4;//image's width
                        $im = imagecreate($width + 40,$height+20);
                        $lineColor = imagecolorallocate($im,12,12,12);
                        $bgColor = imagecolorallocate($im,235,233,233);
                        $tColor = imagecolorallocate($im,123,200,56);
                        imagefill($im,0,0,$bgColor);
                        imageline ( $im, 30, 0, 30, $height - 2, $lineColor);
                        imageline ( $im, 30, $height - 2, $width + 30 -2 , $height - 2,$lineColor);
                        while($i < $num){
                                imagefilledrectangle ( $im, $i * ($tspace+$twidth) + 40, $height - $dataValue[$i], $i * ($tspace+$twidth) + 40 + $twidth, $height - 3, $tColor);
                                imagestringup ( $im, 4, $i * ($tspace+$twidth) + $twidth/2 + 30, $height - 10, $dataName[$i]."(".$dataValue[$i].")", $lineColor);
                                $i++;
                        }
                        while($j <= (500/10)){
                                imagestringup ( $im, 4, 2, $height - $j * 10 + 10, $j * 10, $lineColor);
                                $j = $j + 10;
                        }
                        while($k <= (500/10)){
                                if($k != 0)
                                imageline ( $im, 28, $height - $k * 10, 32 , $height - $k * 10,$lineColor);
                                $k = $k + 10;
                        }
                        imagepng($im);
                }
//调用方法：
$data = array("Yahoo" => 280, "Google" => 260,"Microsoft" => 320,"IBM" => 290,"Sun System" => 150,"Inter" => 260);
for($i=0;$i<count($city);$i++){
//$data=iconv("gb2312","UTF-8",$citypower[$i]);
$data[$city[$i]]=$citypower[$i];}
createImage($data,38,25,460);
?>输出的柱状图上中文是乱码怎么办呢？？？？？？