@landsom - I don't believe that is available in that exact form currently, but there is another method that you can use, although it is a little more tedious.  If you make the activity into a "segment", then you can use the "compare" tool to compare your various efforts against one another.  It doesn't animate quite like the flyby, but you could compare up to I think 5 efforts over the same "segment" at a time.  

If you are not familiar with the "compare" process, here is a quick rundown:  Open one of your activities, look at the segment list and choose the option to "create new segment" at the bottom.  Follow the directions to create the segment covering the length of your activity.  Give it some time to populate the database from past data files.  Open the segment itself and choose "compare efforts" (I believe this is only available on a desktop browser and not in the app).  Sort the segment results so it only shows your efforts and then choose the ones you want to compare from the list.  

I know this isn't quite what you were looking for, but might give you a way to work around it in the meantime.  Hopefully they will add that "flyby" option like you described at some point in the future.

Thanks!

Was not aware of this function, I will start using it right away. 

Thanks for the workaround.  It is more of an effort than if the comparison was built in.  The only problem with creating a segment for the whole activity is that the segment comparison uses elapsed time, whereas the activity matching uses moving time (which differ by how much time I spend taking photos and admiring the view, if I forget to pause the Garmin).  Being able to toggle between the two would be useful