public function addChart(Chart $pChart, $iChartIndex = null)
    {
        $pChart->setWorksheet($this);
        if ($iChartIndex === null) {
            $this->chartCollection[] = $pChart;
        } else {
            // Insert the chart at the requested index
            array_splice($this->chartCollection, $iChartIndex, 0, [$pChart]);
        }

        return $pChart;
    }