def to_json(*args)
      rows = []
      self.each_row do |row|
        rows.push(row.to_hash)
      end
      rows.to_json
    end
def to_json(*args)
      rows = []
      self.each_row do |row|
        rows.push(row.to_hash)
      end
      rows.to_json
    end
Compartible with Nyaplot::DataFrame.to_json