Example:

$workbook = new cells\Workbook();
$cells = $workbook->getWorksheets()->get(0)->getCells();
//Put values in cells
$cells->get("A1")->putValue(1);
$cells->get("A2")->putValue(2);
$cells->get("A3")->putValue(3);
$chartIndex = $workbook->getWorksheets()->get(0)->getCharts()->add(cells\ChartType::COLUMN, 3, 3, 15, 10);
$chart = $workbook->getWorksheets()->get(0)->getCharts()->get($chartIndex);
$chart->getNSeries()->add("A1:a3", true);
$chart->getNSeries()->get(0)->getTrendLines()->add(cells\TrendlineType::LINEAR, "MyTrendLine");
$line = $chart->getNSeries()->get(0)->getTrendLines()->get(0);
$line->setDisplayEquation(true);
$line->setDisplayRSquared(true);
$line->setColor(cells\Color::getRed());