首先，你需要包含（require）jpgraph_plotline.php文件，比如：

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

然后，在$graph->Stroke();行之前，添加绘制该行的代码，例如：

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

因此，整个代码将像这样：

<?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();
?>

这样，绘图线将被添加到折线图上（如下所示-y位置5处的绿色线，宽度：10）

如果需要，只需对PlotBand执行相同的操作。
好好享受...