def update!(data)
      data_str = data.to_gnuplot_points
      if @stored_in_file
        File.open(@file_name, 'a') { |f| f.puts "\n#{data_str}" }
      else
        @data = "#{@data}\n#{data_str}"
      end
      self
    end
def update!(data)
      data_str = data.to_gnuplot_points
      if @stored_in_file
        File.open(@file_name, 'a') { |f| f.puts "\n#{data_str}" }
      else
        @data = "#{@data}\n#{data_str}"
      end
      self
    end
Update existing Datablock with new data.
 Destructive version of #update.

 @param data [#to_gnuplot_points] anything with #to_gnuplot_points method
 @return [Datablock] self

 @example
   data = [[0, 1, 2, 3], [0, 1, 4, 9]] # y = x**2
   db = Datablock.new(data, false)
   update = [[4, 5], [16, 25]]
   db.update!(update)
   # now db contains points with x from 0 up to 5