def delete(label)
      pos = @labels.index(label)
      raise "there is no column named " + label.to_s if pos.nil?
      @labels.delete_at(pos)
      @data.delete_at(pos)
    end
def delete(label)
      pos = @labels.index(label)
      raise "there is no column named " + label.to_s if pos.nil?
      @labels.delete_at(pos)
      @data.delete_at(pos)
    end
Delete column