Add an horizontal line or a band to jpGraph

I'm trying to add a simple horizontal line (or a band) to my graph and for some reasons, it doesn't show anything.

I used the lines:

$graph->AddLine(new PlotLine(HORIZONTAL,0,"black",2));

or

$graph->AddBand(new PlotBand(HORIZONTAL,BAND_RDIAG,0, "max", "red", 2));

...as shown here https://jpgraph.net/download/manuals/classref/Graph.html

Is there anything else I should do? Is there a specific place where I should put it? I put them just after the $graph->Add($lineplot);

Thanks for your help

First of all, you need to include (require) the jpgraph_plotline.php file, such as:

require_once ('jpgraph-4.2.5/src/jpgraph_plotline.php');

Then, before the line $graph->Stroke();, add the code for plotting the line, such as:

// Create a line to add as markers
$l1 = new PlotLine(HORIZONTAL, 5, 'green:1.5', 10);
 // Add lines to the plot
$graph->AddLine($l1);

So, the whole code will be like:

<?php 
require_once ('jpgraph-4.2.5/src/jpgraph.php');
require_once ('jpgraph-4.2.5/src/jpgraph_line.php');
require_once ('jpgraph-4.2.5/src/jpgraph_plotline.php');

$datay1 = array(20,15,23,15);
$datay2 = array(12,9,42,8);
$datay3 = array(5,17,32,24);

// Setup the graph
$graph = new Graph(300,250);
$graph->SetScale("textlin");
$graph->title->Set('StackOverflow.com');
$graph->SetBox(false);
$graph->SetMargin(40,20,36,63);

// Create the first line
$p1 = new LinePlot($datay1);
$graph->Add($p1);
$p1->SetColor("#6495ED");
$p1->SetLegend('Line 1');

// Create the second line
$p2 = new LinePlot($datay2);
$graph->Add($p2);
$p2->SetColor("#B22222");
$p2->SetLegend('Line 2');

// Create the third line
$p3 = new LinePlot($datay3);
$graph->Add($p3);
$p3->SetColor("#FF1493");
$p3->SetLegend('Line 3');

// Create a line to add as markers
$l1 = new PlotLine(HORIZONTAL, 5, 'green:1.5', 10);
 // Add lines to the plot
$graph->AddLine($l1);

// Output the result
$graph->Stroke();
?>

In this way, the plot line will be added on the line chart (as below - a green line at y position 5, width :10)

Just do the same for PlotBand, if you wish.

Enjoy...