docs: def set_label(self, text, location='upper right', style=None):
          """Set a label for the plot.
  
          :param text: the label text.
          :param location: the location of the label inside the plot.  May
              be one of 'center', 'upper right', 'lower right', 'upper
              left', 'lower left'.
          :param style: any TikZ style to style the text.
  
          """
          if location in RELATIVE_NODE_LOCATIONS:
              label = RELATIVE_NODE_LOCATIONS[location].copy()
              label['text'] = text
              label['style'] = style
              self.label = label
          else:
              raise RuntimeError('Unknown label location: %s' % location)