def METHOD_NAME(self, formula, wion_symbol):
    if self.xaxis in {"capacity", "capacity_grav"}:
        return "Capacity (mAh/g)"
    if self.xaxis == "capacity_vol":
        return "Capacity (Ah/l)"
    formula = formula.pop() if len(formula) == 1 else None
    wion_symbol = wion_symbol.pop() if len(wion_symbol) == 1 else None
    if self.xaxis == "x_form":
        if formula and wion_symbol:
            return f"x in {wion_symbol}<sub>x</sub>{formula}"
        return "x Workion Ion per Host F.U."
    if self.xaxis == "frac_x":
        if wion_symbol:
            return f"Atomic Fraction of {wion_symbol}"
        return "Atomic Fraction of Working Ion"
    raise RuntimeError("No xaxis label can be determined")