It is composed of the T wave, which represents repolarization, and the P wave and QRS Complex which correspond to the contraction of the atria and ventricles, respectively.  
Rautaharju and Bazett have both developed methods for calculating the QT interval, a diagnostic tool which derives from this graphic.  It can detect calcium, magnesium, and other electrolyte disturbances and conduction abnormalities.  It is composed of the T wave, which represents repolarization, and the P wave and QRS Complex which correspond to the contraction of the atria and ventricles, respectively.  FTP, name this graphic that can also detect coronary artery blockages and cardiac arrhythmia by recording the electrical voltage of the heart.
Rautaharju and Bazett have both developed methods for calculating the QT interval, a diagnostic tool which derives from this graphic.  
electrocardiogram or ECG or EKG [Elektrokardiogramm]