jpgraph中的例子如何使用鼠标拖动实现图形的缩放？


下面的代码是jpgraph\Examples\accbarex1.php
该段代码能绘出一个柱状图，那么如何使绘出的图能通过鼠标拖动实现图形的缩放呢？
请高手指点，并尽量详细些，本人刚刚入门，多谢各位大侠<?php // content="text/plain; charset=utf-8"require_once ('jpgraph/jpgraph.php');
require_once ('jpgraph/jpgraph_bar.php');$data1y=array(-8,8,9,3,5,6);
$data2y=array(18,2,1,7,5,4);// Create the graph. These two calls are always required
$graph = new Graph(500,400); 
$graph->SetScale("textlin");$graph->SetShadow();
$graph->img->SetMargin(40,30,20,40);// Create the bar plots
$b1plot = new BarPlot($data1y);
$b1plot->SetFillColor("orange");
$b1plot->value->Show();
$b2plot = new BarPlot($data2y);
$b2plot->SetFillColor("blue");
$b2plot->value->Show();// Create the grouped bar plot
$gbplot = new AccBarPlot(array($b1plot,$b2plot));// ...and add it to the graPH
$graph->Add($gbplot);$graph->title->Set("Accumulated bar plots");
$graph->xaxis->title->Set("X-title");
$graph->yaxis->title->Set("Y-title");$graph->title->SetFont(FF_FONT1,FS_BOLD);
$graph->yaxis->title->SetFont(FF_FONT1,FS_BOLD);
$graph->xaxis->title->SetFont(FF_FONT1,FS_BOLD);// Display the graph
$graph->Stroke();
?>