def _format_value_error(self, v, e, pm=" +/- "):
        """
        Returns a string v +/- e with the right number of sig figs.
        """
        # If we have weird stuff
        if not _s.fun.is_a_number(v) or not _s.fun.is_a_number(e) \
            or v in [_n.inf, _n.nan, _n.NAN] or e in [_n.inf, _n.nan, _n.NAN]: 
            return str(v)+pm+str(e)
        
        # Normal values.
        try:
            sig_figs = -int(_n.floor(_n.log10(abs(e))))+1            
            return str(_n.round(v, sig_figs)) + pm + str(_n.round(e, sig_figs))

        except:
            return str(v)+pm+str(e)
Returns a string v +/- e with the right number of sig figs.