As titled.
I am new to Garmin. But the heart rate chart itself isn’t that useful with only the average HR being shown.
can Connect shows something like Polar Flow? 
this way, it is easier to track when during an activity, and for how long a stretch, I am in a certain zone.
Connect’s implementation is too simplistic. 
if I can’t do it, how can I feed that back to the developers?
Not so clearly, but "kind of" you can see it in a similar way as you requested, on the all-day HR graph. Better viewable in Garmin Connect Web, since you can zoom the selected section, there. The coloring…
... and another option where you can see the HR Zones, although not on a time based chart, is the HR layer of the activity map. Personally I find it even more useful than what you request, in this way.
…
Not so clearly, but "kind of" you can see it in a similar way as you requested, on the all-day HR graph. Better viewable in Garmin Connect Web, since you can zoom the selected section, there. The coloring of the graph seems to respect the HR Zones, but I am unsure how well it works if you have several HR Zone tables for different sports defined, and do several sports in the same day.
... and another option where you can see the HR Zones, although not on a time based chart, is the HR layer of the activity map. Personally I find it even more useful than what you request, in this way.
