Stay On Pulse Creating A Shape-Driven Electrocardiogram
You can find EKGs made with After Effects everywhere. The good thing about this one is that it just uses shape layers and mattes to create a realistic effect.
This tutorial will teach you how to create the basic shape of the line and how to make it appear over the screen.
Adapting what you learn here, you can move on to create the whole piece, such as this complete EKG for sale at VideoHive.