I find the fitness trend tracker really frustrating. As far as I can tell it makes an assumption that every day you don't log something in Strava, your fitness declines by x% and also makes assumptions about the incremental fitness you gain from each activity. Given that not everything I do is logged on Strava (shocking, I know) this is totally inaccurate. However, you have all the data you need to work out actual fitness, as, over time, I will do similar workouts, at similar levels of effort and you have all my HR data.  For example, if I do a 5k run in September at a certain speed and HR, and then I do the same run (or similar) 4 months later at the same HR but faster, you know my fitness has increased. You must be able to build an algorithm that calculates some index of fitness by comparing my HR, relative effort, speed etc etc, regardless of how many sessions I have actually logged.