A game is missing
Apps appear once Steam publishes them on PICS and the store poller has seen the store page. New apps usually show up within an hour of their store page going live.
The all-time peak differs from other sites
SteamPeaks only reports what it has sampled itself. Older sites have longer archives; the number here grows from the day tracking started for each game.
Do I need an account?
No. Every table and chart is free without one. Signing in adds a watchlist, alerts, the dashboard and library highlighting.
Is there an API?
CSV exports and RSS feeds are open on every list. A JSON API with keys is planned; the explorer page lists what exists today.