In an $ECG$,the depolarization of atria is indicated by

In the following diagrammatic representation of a standard $ECG$,the '$T$' wave represents:

$X-$ In $ECG,$ $P-$ wave represents the electrical excitation of the ventricle.
$Y -$ $QRS$ waves represent depolarization of atria.

What does an $ECG$ measure?

What does the blood pressure reading $120/80 \ mmHg$ represent?

$ECG$ depicts the depolarization and repolarization processes during the cardiac cycle. In the $ECG$ of a normal healthy individual,one of the following waves is not represented?