Smooth choppy idea for PHP GD plots

I cannot get smooth lines for this graph. I have tried anti-aliasing by zooming in and I found several anti-aliasing functions on php.net but the results are inconsistent and sometimes result in broken lines.

I have UPDATED THE CODE BELOW AND SHOWED / COMMENTS FOR CHANGES USED TO CORRECT THE NUMBER

Code:

<?php
header("Content-type: image/png");

$Values=array(rand(40,80),rand(40,100),rand(10,50),rand(80,160),rand(30,100),rand(40,120),rand(280,360),rand(20,80),rand(10,80),rand(40,120),rand(180,260),rand(40,160),rand(550,700),rand(480,600),rand(240,340),rand(480,600),rand(240,340));
$imgWidth=500;
$imgHeight=200;
$grid=25;
$graphspacing=0.05;

while (list($key, $val) = each($Values)) {
    if($val>$max){
        $max=$val;
    }
}

for ($i=0; $i<count($Values); $i++){
    $graphValues[$i] = $Values[$i] * (($imgHeight*(1-$graphspacing))/$max);
}

// use imagecreatetruecolor instead of imagecreate
$image=imagecreatetruecolor($imgWidth, $imgHeight);

// added antialiasing
imageantialias($image, true);

// had to force a white bg
$bgColor = imagecolorallocate($image, 255, 255, 255);
imagefill($image , 0,0 , $bgColor);

$colorWhite=imagecolorallocate($image, 255, 255, 255);
$colorGrey=imagecolorallocate($image, 192, 192, 192);
$colorBlue=imagecolorallocate($image, 0, 0, 255);

imageline($image, 0, 0, 0, $imgHeight, $colorGrey);
imageline($image, 0, 0, $imgWidth, 0, $colorGrey);
imageline($image, $imgWidth-1, 0, $imgWidth-1, $imgHeight-1, $colorGrey);
imageline($image, 0, $imgHeight-1, $imgWidth-1, $imgHeight-1, $colorGrey);

// Create grid
for ($i=1; $i<($imgWidth/$grid); $i++) {
    imageline($image, $i*$grid, 0, $i*$grid, $imgHeight, $colorGrey);
}
for ($i=1; $i<($imgHeight/$grid); $i++) {
    imageline($image, 0, $i*$grid, $imgWidth, $i*$grid, $colorGrey);
}

if($imgWidth/$grid>count($graphValues)){ 
    $space=$grid; 
} else { 
    $space = $imgWidth/(count($graphValues)-1);
}

for ($i=0; $i<count($graphValues)-1; $i++) {
    imageline($image, $i*$space, ($imgHeight-$graphValues[$i]), ($i+1)*$space, ($imgHeight-$graphValues[$i+1]), $colorBlue);
}

      

        
        
        
      

    

// added smooth filter imagefilter ($ image, IMG_FILTER_SMOOTH, 15);

imagepng($image);//,NULL,9);
imagedestroy($image);

?>

      

        
        
        
      

    

Result (best I got):

before

after

Cheers.Bo