Chartjs-Yii2 Chart is not defined

To display chart you must echo result of widget

echo Chart::widget([
    'type' => Chart::TYPE_BAR,
    'datasets' => [
        [
            'data' => [
                'Label 1' => 10,
                'Label 2' => 20,
                'Label 3' => 30
            ]
        ]
    ]
]);