Basic rules
Do
Compare category magnitudes on a common scale and start the value axis at zero when length encodes value.
State the unit and sort order and reserve enough room for category labels.
Keep series order, colors and units consistent within each group for like-for-like comparison.
Don't
Do not truncate the baseline to exaggerate differences or vary bar thickness between categories.
Do not rely only on color to identify categories or overlap an excessive number of bars.
Do not crowd too many series into a group until individual bars become unreadable.