add column_group helper

  return std::to_string(v);

}

std::vector<std::pair<size_t, size_t>> column_group(const DataColumn& col) {

  std::vector<std::pair<size_t, size_t>> groups;

  for (size_t idx = 0; idx < col.data.size(); ) {

    auto begin = idx;

    auto end = idx;

    while (end < col.data.size() && col.data[end] == col.data[begin]) {

      end = idx++;

    }

    groups.emplace_back(begin, end);

  }

  return groups;

}

} // namespace plotfx

    const std::string& column_name,

    const DataColumn** column);

std::vector<std::pair<size_t, size_t>> column_group(const DataColumn& col);

size_t series_len(const Series& s);

std::vector<double> series_to_float(const Series& s);