private function connectPoints()
    {
        foreach ($this->points as $point) {

            if (is_null($point->value)) {
                $last_x = $point->x;
                $last_y = $point->y;
                $last_val = $point->value;
                continue;
            }

            if (isset($last_x) && (isset($last_val) && !is_null($last_val))) {
                imageline($this->im, $last_x, $last_y, $point->x, $point->y,
                    $this->ink['line']);
            }
            $last_val = $point->value;
            $last_x = $point->x;
            $last_y = $point->y;
        }
    }