Hello @emmseegee 

Thanks for your post. Can you clarify if his activity is a distance based activity or a time only based activity (zero distance)?

The one thing I notice about the Garmin chart is that the x axis is showing time, whereas on the Strava chart the x axis is showing distance which appears to be zero. 

I'm curious if you've tried looking at this activity on the Web by signing in at strava.com. In the Web view it's possible to switch the heart rate graph between distance and time, and I would be curious to know how it appears if you swith it to time.