The heart rate chart is not displayed if insufficient data has been recorded.
The following steps may help:
On your iPhone, in the Watch app, go to: My Watch (tab) > Privacy > if they are not already enabled, turn on Heart Rate and Fitness Tracking.
On your iPhone, in the Watch app, go to: My Watch (tab) > Workout > turn off Workout Power Saving Mode.
Otherwise, note that Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
For the best performance from the heart rate sensor during workouts, ensure that you wear your Apple Watch snugly (but not too tightly) on the top of your wrist. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards:
The sensor is likely to give better results for workouts that involve rhythmic (eg running) rather than irregular (eg boxing) movements. Other issues that can impact sensor performance include skin perfusion (which can also vary, depending on ambient temperature) and wrist tattoos:
- Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
- Get the most accurate measurements using your Apple Watch - Apple Support
For any workouts that involve flexing your wrists (such as when lifting weights, for example), it may help to move your watch a little higher up your arm, away from the flexing joint.
It may also help to check that the back of your Apple Watch is clean (along with your wrist, which I'm sure is the case):
If you continue to experience issues with recording your heart rate during workouts, then, for more consistent readings, you may wish to consider pairing an external heart rate monitor / Bluetooth chest strap to your Apple Watch:
If you are concerned about a possible hardware defect with your Apple Watch, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: