Heart Graph on iPhone has two different modes, depending on the type of heart rate sensor you are using to measure your heart rate (note that on iPad only Bluetooth sensors are supported). The default mode selection is set when you first launch Heart Graph, but can always be changed at any point in the app settings.

If you’re using an Apple Watch, you should set Heart Graph to be in “Apple Watch” mode:

if you’re using a Bluetooth or ANT+ sensor, you should choose the “BTLE/ANT+” mode:

It’s important to select the correct mode to allow Heart Graph to connect to your chosen sensor – the app will not try to connect to the Apple Watch if you have selected “BTLE/ANT+” mode, and similarly, the app will not try to connect to Bluetooth heart rate sensors if you have chosen “Apple Watch” mode..