Hello,
The smooth option is only available for scatter charts, you can read about this option on:
http://www.phpdocx.com/api-documentation/word-content/add-chart-Word-document-with-PHP
Anyway, you can add support for line charts easily:
1. Edit the file CreateLineChart.inc and around line 169 add these lines:
            if (!empty($this->_smooth) && $this->_smooth && $this->_symbol != 'dot') {
                $this->generateSMOOTH();
            }
Instead of this code:
            foreach ($this->values as $legend => $value) {
                if ($legend == 'legend') {
                    continue;
                }
                $this->generatePT($num);
                $this->generateV($value[$i]);
                $num++;
            }
            $this->cleanTemplate3();
 
The class must use this code:
            foreach ($this->values as $legend => $value) {
                if ($legend == 'legend') {
                    continue;
                }
                $this->generatePT($num);
                $this->generateV($value[$i]);
                $num++;
            }
            if (!empty($this->_smooth) && $this->_smooth && $this->_symbol != 'dot') {
                $this->generateSMOOTH();
            }
            $this->cleanTemplate3();
2. Use the new option (smooth option) when adding the chart:
$paramsChart = array(
    'data' => $data,
    'type' => 'lineChart',
    'color' => '5',
    'chartAlign' => 'center',
    'showTable' => 0,
    'sizeX' => '12',
    'sizeY' => '10',
    'legendPos' => 'b',
    'legendOverlay' => '0',
    'haxLabel' => 'X Axis',
    'vaxLabel' => 'Y Axis',
    'haxLabelDisplay' => 'horizontal',
    'vaxLabelDisplay' => 'vertical',
    'hgrid' => '3',
    'vgrid' => '1',
    'smooth' => true,
);
$docx->addChart($paramsChart);
Regards.