Try setting the scale value, for eg:
Code:
$AxisBoundaries = array(0=>array("Min"=>0,"Max"=>100));
$ScaleSettings = array("Mode"=>SCALE_MODE_MANUAL,"ManualScale"=>$AxisBoundaries,"DrawSubTicks"=>TRUE,"DrawArrows"=>TRUE,"ArrowSize"=>6);
$myPicture->drawScale($ScaleSettings);
You can also have a look at:
http://wiki.pchart.net/doc.doc.draw.scale.html