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.
Make each dot represent an actual observation and provide access to its exact value.
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 draw large markers on every dense observation until they obscure the line.