Basic rules
Do
Show magnitude and change along an ordered axis, with a clear baseline for the filled area.
Label axis units and aggregation intervals and distinguish missing data from actual zero.
Use steps when a value changes at a specific point and holds until the next change.
Don't
Do not connect unordered categories or let overlapping areas completely obscure another series.
Do not fill gaps without explanation and imply continuous observation.
Do not imply a continuously varying measure stays constant between observations without explanation.