When lines drawn with imagesetstyle seem to produce a thin white line only, make sure antialiasing is disabled.
<?
imageantialias($im, false);
$style = array($gridxcolor, $gridxcolor, IMG_COLOR_TRANSPARENT, IMG_COLOR_TRANSPARENT);
imagesetstyle($im, $style);
imageline($im, $x, 0, $x, $ymax+5, IMG_COLOR_STYLED);
imageantialias($im, true);
?>
Setstyle and Antialias don't go together.