It can happen if a satellite has sent no disk usage reports during that day. We’ve seen it happen before. It’s not really an issue as the disk usage gets reported the next day anyway. It’s just a display issue. But at the moment there is also a bug with that graph as it doesn’t display the correct information to begin with. I see the missed day on my nodes as well.
Alexey mentioned he notified the team about an issue with this graph here: Dashboard: Calculation error for "Disk space used This Month" and "Disk Average Month"? - #28 by Alexey