Just my explanation as I work with GM charts. Temperature is in kelvin so the formula is C = K - 273.15 when producing a chart.
If a frame is faulty or missing, which does happen, then the K grid can get filled with -1s or 0’s , which if you don’t account for will give you lots of -273c conversions! I usually deal with this by carrying across the last good frame, which at least prevents a ridiculous looking graph!