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Pulse Oximeter can be a Plethysmograph

A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains). A pulse oximeter can mesure the change in the volume of arterial blood with each pulse beat. This change in blood volume can be detected in peripheral parts of the body such as the fingertip or ear lobe using a technique called photoplethysmography. The pulse oximeter that detects the signal is called a plethysmograph (or ‘Pleth’ for short).

The blood flow is normally shown as a wareform using a bar or graph. It can provide useful information regarding the heart condition.

Only more advanced pulse oximeter would provide the waveform. Read this article "Photoplethysmograph" for more information on this topic.