I agree that this would be a useful facility - in fact, as a runner and cyclist, I'd prefer it as the default - but it clearly isn't there in the Mobile App. I've usually found that outdoor runners focus on distance, related to known landmarks or mile / km markers in events, and indoor, machine runners focus more on time. GPS devices must be more useful to outdoor runners who I would have thought would mainly expect distance based graphs. That allows them to see how fast they were going on a particular straight level stretch or what their heart rate was up a hill at some known point on the route.
Perhaps I'm using the graphs wrongly and they're really useful when based on time, but either way it would be nice to have the option in the Mobile App.