Basic rules
Do
Show trends along an ordered axis such as time and clarify observation intervals and units.
Use a shared scale for comparable series and distinguish missing values from actual zero.
Treat smoothing as a visual aid for the trend and make actual observations available.
Don't
Do not connect unordered categories and imply a trend.
Do not hide axis breaks or irregular observation intervals that change how slopes are read.
Do not let interpolation imply unobserved extremes or values outside the valid data range.