code: def save_graph(graph_str, dest_file, fmt=None, image_ratio=None):
      
      g = pydot.graph_from_dot_data(graph_str)
  
      # determine the dest format
      if fmt is None:
          fmt = os.path.splitext(dest_file)[1].lower().strip('.') or "png"
      if hasattr(g, "write_" + fmt):
          write_fn = getattr(g, "write_" + fmt)
      else:
          raise Exception("Unsupported graph format: '%s'" % fmt)
  
      if image_ratio:
          g.set_ratio(str(image_ratio))
      write_fn(dest_file)
      return fmt
docs: Render a graph to an image file.
  
      Args:
          graph_str (str): Dot-language graph string.
          dest_file (str): Filepath to save the graph to.
          fmt (str): Format, eg "png", "jpg".
          image_ratio (float): Image ratio.
  
      Returns:
          String representing format that was written, such as 'png'.