A dictionary of dictionaries.
        Each dictionary defines a map which is used to extend the metadata.

        The precise way maps interact with the metadata is defined by `figure.fit._extend_meta`.
        That method should be redefined or extended to suit specific use cases.

def maps(self):
        """
        A dictionary of dictionaries.
        Each dictionary defines a map which is used to extend the metadata.

        The precise way maps interact with the metadata is defined by `figure.fit._extend_meta`.
        That method should be redefined or extended to suit specific use cases.
        """
        if not hasattr(self, '_maps'):
            maps = {}
            maps['tex_symbol'] = {}
            maps['siunitx'] = {}
            maps['value_transforms'] = {
                '__default__': lambda x: round(x, 2),
            }
            self._maps = maps
        return self._maps