We can draw vertical grid to the graph we developed in the basic line graph tutorial. To draw a vertical grid line from each point we have to draw line from graph point to the X axis. Note that the X-axis value from starting and ending point of the grid does not change as the lines are vertical. So x1 and x2 value ( starting and end point ) of x axis is same. The Y axis value starts from the graph point ( y value of the month ) to zero line of our graph and that is the point where Y value is maximum ( the horizontal line at the bottom ). So Y will vary from graph point to $y_max setting of the image. So to draw the vertical grid line we have to add one line only.

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

Of course we have to set the value of $grid_color before this line. We will get image like this. For a better clarity than previous tutorial on basic of line graph we have reduced the height of the graph.

The two lines added here are marked in bold letters.

<?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//

$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");