I'm currently working on a holter monitor. I can successfully show the heart rate (bpm) using the Heart Rate Measurement (0x2A37) characteristic of Bluetooth. But I want to show an ECG line graph so I need the raw heart data, with the voltage amounts specified. I can reach some other information such as RR-interval and sensor contact status (with this characteristic) but I don't know how to get the raw data itself. Is it possible that I'm missing a feature of this characteristic, or is there any other way I can get this data? I'm using an AD8232 heart monitor.
The GATT Heart Rate Service doesn't provide access to the raw signal (see Gatt specifications).
If you have access to the hardware platform you will have to write a custom service and characteristic that supports notification, and forward the data from the sensor onto this. You'll then have to write the matching app-side code to get this data and convert it into a usable format.