def truncate_data(offset)
      for t in ALL_COLUMNS
        times = @times[t]
        @times[t] = @times[t][offset, times.size]
        @repeat = @times[t].size
      end
      @analysis = nil
      self
    end
def truncate_data(offset)
      for t in ALL_COLUMNS
        times = @times[t]
        @times[t] = @times[t][offset, times.size]
        @repeat = @times[t].size
      end
      @analysis = nil
      self
    end
Truncate the measurements stored in this clock starting from the integer
 +offset+.