When using the following sketch:
void setup() {
  Serial.begin(9600);
  while (!Serial);
  Serial.println("aX,aY,aZ");
  Serial.println(); // <-- without this line the labels are fine
}
void loop() {
  Serial.print("0,0,0");
  Serial.println();
  delay(1000);
}
The aX label is lost in the Serial plotter:

If the Serial.println(); line in setup is removed, all is good.