The innocent axis
Change the y-axis and the same movement can look dramatic or trivial. Aggregate categories differently and a pattern can vanish. Smooth too much and you hide variation; smooth too little and you amplify noise.
Charts make arguments
A chart is not just a picture of data. It is a set of decisions about scale, comparison, emphasis and omission. Those decisions guide what the reader notices.
A useful checklist
Ask: what is the denominator, what is missing, what comparison is being encouraged, and would the conclusion survive another reasonable chart?