def integral()
      begin
        sum = 0
        Histogram.new(
          points.map do |(x, y)|
            sum += y
            [x, sum]
          end
        )
      end
    end
def integral()
      begin
        sum = 0
        Histogram.new(
          points.map do |(x, y)|
            sum += y
            [x, sum]
          end
        )
      end
    end
literal CDF