On the pulse signal chart the X-axis units are seconds.
The Y-axis values are non-calibrated units.
It’s the raw voltage signal from the pulse sensor recorded by the analog to digital converter (ADC). The pulse transducer (sensor) uses infra-red (IR) light to measure changes in blood density each time the heart beats, the photo diode converts the IR light modulations to a voltage and the voltage is converted to a digital signal by the ADC Y-axis.
Calibration is not required because we are measuring the time between pulses not the pulse themselves. The Y-axis units are just the raw voltage steps of the ADC.