Create the formatted string that displays the progress.

def format_progress(self, width):
        """Create the formatted string that displays the progress."""
        chunk_widths = self._get_chunk_sizes(width)
        progress_chunks = [chunk.format_chunk(chunk_width)
                           for (chunk, chunk_width)
                           in zip(self._progress_chunks, chunk_widths)]
        return "{sep_start}{progress}{sep_end}".format(
            sep_start=self.sep_start,
            progress="".join(progress_chunks),
            sep_end=self.sep_end
        )