BTW, should mention to watch units. That chart is given in °C, so when you drop below 50% state of charge the possibility does start to increase quite dramatically. At 25% SoC the point that particular curve indicates freezing could occur at -4°F and it wouldn't take but a day for the mass of a battery to equalize. A dead battery will freeze like it's water, at 32°F. But >75% SoC it would take interior of Alaska (-58°F) to start to worry.
Also, if you suspect you have a frozen battery and it has not split the case bring it inside and let it warm up gently, preferably in a tub or something just in case it does leak. Don't attempt to accelerate and definitely do not put it on a charger or attempt a jump start. Frozen electrolyte doesn't work and pushing high current through it can cause an explosion.