Save plot assets alongside dest_path.

        Some plots may have assets, like bitmap files, which need to be
        saved alongside the rendered plot file.

        :param dest_path: path of the main output file.

def save_assets(self, dest_path):
        """Save plot assets alongside dest_path.

        Some plots may have assets, like bitmap files, which need to be
        saved alongside the rendered plot file.

        :param dest_path: path of the main output file.

        """
        for idx, subplot in enumerate(self.subplots):
            subplot.save_assets(dest_path, suffix='_%d' % idx)