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.
Connect observations with straight segments and distinguish interpolated segments from measured values.
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 describe unobserved intermediate values as measurements.