d3.csv(path).then((data) => {
  // get columns name
  data.columns
  
  // get columns name except for some columns
  columns = data.columns.filter(d => d !== 'class' && d !== 'id' && d !== '');
  
  // select certain one column as array
  key_data = data.map(d => (d[key]));
  
  // select certain columns as array of object
  keys_data = data.map(d => ({key1: d[key1], key2: d[key2])});
})