Drawing a bar graph based on records in the database

Hullo,

I have hit a snag when I got to the point where I draw a bar graph.
I want to have a bar graph based on the database records, but I have no idea on how to do it.
I tried searching online, but to no avail. I only found a code that is static; the one below;

<?php
    # ------- The graph values in the form of associative array
    $sql="SELECT * FROM marks WHERE school_id='14'";
    $result=mysql_query($sql);
    $row = mysql_fetch_array($result);

    $subject = $row['subject_name'];
    $mark = $row['subject_score'];

    $values=array($subject => $mark);

    $img_width=450;
    $img_height=300; 
    $margins=20;


    # ---- Find the size of graph by substracting the size of borders
    $graph_width=$img_width - $margins * 2;
    $graph_height=$img_height - $margins * 2; 
    $img=imagecreate($img_width,$img_height);


    $bar_width=20;
    $total_bars=count($values);
    $gap= ($graph_width- $total_bars * $bar_width ) / ($total_bars +1);


    # -------  Define Colors ----------------
    $bar_color=imagecolorallocate($img,0,64,128);
    $background_color=imagecolorallocate($img,240,240,255);
    $border_color=imagecolorallocate($img,200,200,200);
    $line_color=imagecolorallocate($img,220,220,220);

    # ------ Create the border around the graph ------

    imagefilledrectangle($img,1,1,$img_width-2,$img_height-2,$border_color);
    imagefilledrectangle($img,$margins,$margins,$img_width-1-$margins,$img_height-1-$margins,$background_color);


    # ------- Max value is required to adjust the scale -------
    $max_value=max($values);
    $ratio= $graph_height/$max_value;


    # -------- Create scale and draw horizontal lines  --------
    $horizontal_lines=20;
    $horizontal_gap=$graph_height/$horizontal_lines;

    for($i=1;$i<=$horizontal_lines;$i++){
        $y=$img_height - $margins - $horizontal_gap * $i ;
        imageline($img,$margins,$y,$img_width-$margins,$y,$line_color);
        $v=intval($horizontal_gap * $i /$ratio);
        imagestring($img,0,5,$y-5,$v,$bar_color);

    }


    # ----------- Draw the bars here ------
    for($i=0;$i< $total_bars; $i++){ 
        # ------ Extract key and value pair from the current pointer position
        list($key,$value)=each($values); 
        $x1= $margins + $gap + $i * ($gap+$bar_width) ;
        $x2= $x1 + $bar_width; 
        $y1=$margins +$graph_height- intval($value * $ratio) ;
        $y2=$img_height-$margins;
        imagestring($img,0,$x1+3,$y1-10,$value,$bar_color);
        imagestring($img,0,$x1+3,$img_height-15,$key,$bar_color);       
        imagefilledrectangle($img,$x1,$y1,$x2,$y2,$bar_color);
    }
    header("Content-type:image/png");
    imagepng($img);

?>

Only problem is that this only returns one subject on the graph.