# File lib/powerbar.rb, line 394
    def append(at, v)
      return if @sample_interval > at - @last_sample_at
      @sample_interval += @sample_interval_step_up if @sample_interval < @sample_interval_max

      rate = (v - @counter) / (at - @last_sample_at).to_f
      return if rate.nan?

      @last_sample_at = at
      @counter = v

      self << rate
      shift while @window < length

      self
    end