protected function getSparkline($metric)
{
// sparkline is always echoed, so we need to buffer the output
$view = $this->getRowEvolutionGraph($graphType = 'sparkline', $metrics = array($metric => $metric));
ob_start();
$view->render();
$spark = ob_get_contents();
ob_end_clean();
// undo header change by sparkline renderer
Common::sendHeader('Content-type: text/html');
// base64 encode the image and put it in an img tag
$spark = base64_encode($spark);
return '<img src="data:image/png;base64,' . $spark . '" />';
}