Weird, because whenever I have Tor Browser opened, it does have an uploading and downloading bandwidth.
I believe it is just a regular internet traffic graph over Tor.
That's with just one window, at the main "most private browsing experience" page. I presume these bytes transferred are like heartbeats?
It could be reachability test or circuit build process. If you slide next to Connections page in
nyx you will see that there are already Tor circuits and the one that is still being built("Building..."). The "heartbeats" you talk could be the process of building it or checking the circuit timeout[1].
[1]
https://tor.stackexchange.com/a/4636
