Looking quickly at some other posts, looks like you might need to clear the browser cache.
You might want to check out this forum https://forums.garmin.com/showthread.php?t=7217
Here is a screenshot of my settings to reset Safari. After you reset all cookies you will have to manually re-enter all of your old passwords. Autofill will still work if you have that enabled. After the reset the graphs should appear. We are using the same version of Safari.