def archived_attribute_delta_by_day(attribute, start_time = DEFAULT_DAYS_START.ago, end_time = DEFAULT_DAYS_END.ago)
      archived_attribute_by_day(attribute, start_time, end_time) do |days, interpolation|
        days.map.with_index(1) do |d, i| 
          [Time.at(d).strftime('%Y-%m-%d'), interpolation.at(days[i].to_i) - interpolation.at(d.to_i) ] if days[i]
        end
      end
    end
def archived_attribute_delta_by_day(attribute, start_time = DEFAULT_DAYS_START.ago, end_time = DEFAULT_DAYS_END.ago)
      archived_attribute_by_day(attribute, start_time, end_time) do |days, interpolation|
        days.map.with_index(1) do |d, i| 
          [Time.at(d).strftime('%Y-%m-%d'), interpolation.at(days[i].to_i) - interpolation.at(d.to_i) ] if days[i]
        end
      end
    end
returns a hash in which each value is a *delta* of values