def to_yaml_object
      return nil if @categories.empty?
      @categories.each do |cat, info|
        info[:stddev] = stddev(cat)
        info[:median] = median(cat) if info[:buckets]
        info[:interval_95_percent] = percentile_interval(cat, 95) if info[:buckets]
      end
      @categories
    end
def to_yaml_object
      return nil if @categories.empty?
      @categories.each do |cat, info|
        info[:stddev] = stddev(cat)
        info[:median] = median(cat) if info[:buckets]
        info[:interval_95_percent] = percentile_interval(cat, 95) if info[:buckets]
      end
      @categories
    end
Returns all the categories and the tracked duration as a hash than can be exported to YAML