beat-by-beat heart rate data from an Apple Watch?

This the default settings,when you take an ECG you place your finger on the digital crown for 30 seconds and that pic I posted is the end result,you can read more about it here;

Taking an ECG with the ECG app on Apple Watch Series 4 or later – Apple Support

The closest thing to what you ask is to export an ECG reading from the ECG app in .pdf format like this;

Take an ECG from your Apple Watch.

Go to the health app on the phone

tap on browse

tap on heart

tap on ecg

tap on sinus rhythm

scroll down to export pdf to your doctor.

Like this;