# File lib/gruff/base.rb, line 542
    def normalize(force=false)
      if @norm_data.nil? || force
        @norm_data = []
        return unless @has_data

        calculate_spread

        @data.each do |data_row|
          norm_data_points = []
          data_row[DATA_VALUES_INDEX].each do |data_point|
            if data_point.nil?
              norm_data_points << nil
            else
              norm_data_points << ((data_point.to_f - @minimum_value.to_f ) / @spread)
            end
          end
          @norm_data << [data_row[DATA_LABEL_INDEX], norm_data_points, data_row[DATA_COLOR_INDEX]]
        end
      end
    end