Presence, minute by minute
Each machine reports once a minute: who is signed in, and when input last happened. A gap shorter than your idle threshold counts as work; a longer one counts as idle — all of it, retroactively, once the length of the gap is known.
A day you can read
Working hours per day as a calendar heatmap, the shape of the day at fifteen-minute resolution, and the day itself as plain intervals: 09:12–11:40 work, 11:40–12:15 idle.
Nothing you did not ask for
No screenshots. No window titles. No list of running programs. The difference between «nobody was there» and «someone was there and not working» is visible; what they were working on is not.