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.
Identify each series in the legend and keep axis and legend states consistent when a series is hidden.
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 exaggerate relationships between differently scaled series with unexplained dual axes.