Plotting vertical grid to line graph

imageline ($im,$x2, $y_max,$x2,$y2,$grid_color);

<?php
include "db.php";

$qt=mysql_query("select * from gd_graph");

$x_gap=40; // The gap between each point in y axis 

$x_max=$x_gap*13; // Maximum width of the graph or horizontal axis
$y_max=160; // Maximum hight of the graph or vertical axis
// Above two variables will be used to create a canvas of the image//

$im = @ImageCreate ($x_max, $y_max);
or die ("Cannot Initialize new GD image stream");
$background_color = ImageColorAllocate ($im, 234, 234, 234);
$text_color = ImageColorAllocate ($im, 233, 14, 91);
$graph_color = ImageColorAllocate ($im,25,25,25);

$grid_color = ImageColorAllocate ($im,125,125,125);

$x1=0;
$y1=0;
$first_one="yes";
while($nt=mysql_fetch_array($qt)){
//echo "$nt[month], $nt[sales]";
$x2=$x1+$x_gap; // Shifting in X axis
$y2=$y_max-$nt[sales]; // Coordinate of Y axis
ImageString($im,2,$x2,$y2,$nt[month],$graph_color);

// Drawing the grid vertically//
imageline ($im,$x2, $y_max,$x2,$y2,$grid_color); 

//Line above is to print month names on the graph
if($first_one=="no"){ // this is to prevent from starting $x1= and $y1=0
imageline ($im,$x1, $y1,$x2,$y2,$text_color); // Drawing the line between two points//
}
$x1=$x2; // Storing the value for next draw
$y1=$y2;
$first_one="no"; // Now flag is set to allow the drawing
}
//header ("Content-type: image/jpg");

ImageJPEG ($im);
?>