Update Chart.php

This change will allow users to optionally add a Min/Max value to the yAxis, per the following:

//Set yAxis Minimum Value
$yAxisMinValue = 90;
		
//Set yAxis Maximum Value
$yAxisMaxValue = 250;

//	Create the chart
$chart = new PHPExcel_Chart(
	'chart1',		// name
	$title,			// title
	$legend,		// legend
	$plotarea,		// plotArea
	true,			// plotVisibleOnly
	0,				// displayBlanksAs
	NULL,			// xAxisLabel
	$yAxisLabel,		// yAxisLabel
        $yAxisMinValue,         // yAxisMinValue
        $yAxisMaxValue,         // AxisMaxValue				
);