What is a proximity sensor?
A sensor of this type measures distance. The proximity sensor is typically located next to the speaker of a phone and used to turn the display off when the user is using the telephone.
Most phones come with very cheap proximity sensors that can only distinguish between a NEAR ("covered") and a FAR ("not covered") state. Such sensors will only report to values: zero and whatever they consider to be FAR (in centimeters). It is possible that the readout of this sensor will overlap with the X-Axis, making it hard to see.