This is NOT my answer, this is the answer of @Gerben, thank you.
The Arduino IDE suspends the serial connection while uploading, and then resumes when done. It looks like they added some delay before reopening the serial connection. So it looks like your code will run twice, even when you keep the Serial Console open
I suspect (based on your description) that you've got the sensor wired up wrong. You need to include a resistor (aka RL) to convert the change in resistance of the gas sensor to a voltage (in addition to separately powering the heater). See the attached circuit from the datasheet. You could use a fixed resistor of ~20k in place of RL and see how that works ...
The X-axis is fixed and nothing gets plotted on the X-axis. It has its own divisions and they are automatically set by the Arduino IDE. The Arduino IDE still has to do some improvements to its software so that the X- axis can be adjusted and plotted too! This is my experience so far and corrections are welcome!