protected function populateClasses()
    {
        $this->classes = [];
        $cData = count($this->data);

        for ($i = 0; $i < $cData; ++$i) {
            $data = $this->data[$i];

            foreach ($data as $key => $value) {
                if (array_key_exists($key, $this->classes)) {
                    if (array_search($value, $this->classes[$key]) === false) {
                        array_push($this->classes[$key], $value);
                    }
                } else {
                    $this->classes[$key] = [$value];
                }
            }
        }
    }