Overload isset in ChartOption in order to work along with get and set overloaded methods

I overload the isset() method for ChartOption.php class
The get() method overloading in the same class was causing an issue in variable name declaration for javascript at Highchart.php class.

line 24: $chartJS .= "\n    var " . (isset($this->chart->renderTo) ? $this->chart->renderTo : 'chart') . " = new Highcharts.Chart({\n";