add missing configuration check

  }

  /* check configuration */

  if (c->x.size() != c->y.size()) {

  if (c->x.empty() || c->y.empty() || c->x.size() != c->y.size()) {

    return error(

        ERROR,

        "the length of the 'data-x' and 'data-y' properties must be equal");

        "the length of the 'data-x' and 'data-y' properties must be equal and non-empty");

  }

  return labels_draw(ctx, c);