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 the gradient only to support area identification; read values from the line and axes.
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 gradient intensity encodes a value or confidence level.