Annotation

An annotation is a note placed on a point of a chart to say what happened outside the data at that moment — a release, a campaign, an outage.

An annotation is the team's memory on the chart: the chart says 'views jumped here', the annotation says 'because that day such-and-such story published'. Without these notes, three months later no one recalls where that jump or dip came from, and the same analysis is redone from scratch. Its real value is cumulative: over time the chart fills with markers of cause, and patterns (that every big release raises the error rate for a few days, say) reveal themselves. But an annotation is not a causal claim; it only says these two things coincided, not that one produced the other. A good annotation is short, precise and clearly dated, or it becomes useless clutter.