code: def get_label(key):
      
      if key in rel:
          if mpl.rcParams['text.usetex']:
              return rel[key]['label_latex']
          else:
              return rel[key]['label_mpl']
      else:
          return key
docs: Convenience function: return the label (latex version) of a given key,
      if available. Otherwise, return the key itself.