def change_option(self, option_name, new_value):
        """
        Change a config option.

        This function is called if sig_option_changed is received. If the
        option changed is the dataframe format, then the leading '%' character
        is stripped (because it can't be stored in the user config). Then,
        the signal is emitted again, so that the new value is saved in the
        user config.
        """
        if option_name == 'dataframe_format':
            assert new_value.startswith('%')
            new_value = new_value[1:]
        self.sig_option_changed.emit(option_name, new_value)