def chromatogram(chromatogram_id)
      if @index[:chromatogram].has_key? chromatogram_id
        self.seek @index[:chromatogram][chromatogram_id]
        return MzML::Chromatogram.new(parse_next)
      else
        raise MzML::BadIdentifier.new("Invalid ID '#{chromatogram_id}'")
      end
    end
def chromatogram(chromatogram_id)
      if @index[:chromatogram].has_key? chromatogram_id
        self.seek @index[:chromatogram][chromatogram_id]
        return MzML::Chromatogram.new(parse_next)
      else
        raise MzML::BadIdentifier.new("Invalid ID '#{chromatogram_id}'")
      end
    end
Fetch a {MzML::Chromatogram} from the file, given the identifier
 @param chromatogram_id String
 @return {MzML::Chromatogram}