Method is_empty_iterator

source/matplot/util/common.h:639–641

            std::size_t _col_offset;

            static bool is_empty_iterator(const iterator &it) {
                return it._vec == nullptr;
            }
            static bool is_end_iterator(const iterator &it) {
                return is_empty_iterator(it) ||
                       it._row_offset * it._col_offset >= it._vec->size();