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.
Use steps for values held between changes, such as a set price or inventory state.
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 place a step change at a time different from the recorded change.