Hey,
I have a rpoblem with a graph taht shows a temperature. There are not the wohle time values during a day. Most time its “0.0”. So graphana connects the both point - the last value > 0 with the first value > 0.
I know theres an option " Stacking and null value" but it doesn’t matter what is choosen there.
I’ve a second panel and there it works. I don’t have a clue why it works in one panel and not in the other 
Josef