def to_percentage()
      @to_percentage ||=
        begin
          sum = points.sum(&:last)
          Histogram.new(
            points.map do |(x,y)|
              [
                x,
                y.fdiv(sum)
              ]
            end
          )
        end
    end
def to_percentage()
      @to_percentage ||=
        begin
          sum = points.sum(&:last)
          Histogram.new(
            points.map do |(x,y)|
              [
                x,
                y.fdiv(sum)
              ]
            end
          )
        end
    end
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