Shop daily
Three numbers a day: takings, number of sales, cost. Average sale, margin, the seven-day line, which weekday is best and how this week compares to last are all worked out from those three. The books stay on this device — it uses the app's own private store, not a public board, so nobody else can read them or write to them.
Every Monday in one pile, every Tuesday in another, each averaged. The more days you log the truer it gets — this is the chart that tells you when to stock up.
Why the seven-day line matters more than any single bar: one day's high or low is mostly weather and weekends, and tells you nothing about whether the business is getting better or worse. The average flattens seven days out, and what is left is the trend. Same idea behind the weekday chart — it is not what one Saturday took, it is every Saturday averaged, which a single lucky night cannot fake.