The ambient temperature, aka the outside air temperature, isn't 0 degrees Fahrenheit. This is the reason why the oven isn't at 0 degrees Fahrenheit when the oven is off. For example, if it is 68 degrees Fahrenheit outside, then the oven's temperature is likely around this figure when it is off.
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The way the oven works is that it heats the air using a gas or electric source. Once reaching 350 or slightly higher, the oven burner will turn off for a set period of time. This is so the temperature doesn't go too far over 350.
After the burner is turned off, the air inside the oven is still really hot and will stay around 350 for a while. But the temperature will slowly decrease. Once dipping slightly below 350, the oven will turn back on to heat back up to 350 again. This process repeats over and over.
This explains why you see the oscillating bouncing curve like this. It also explains why ovens seem to go on or off in the middle of a baking session.