How people read charts

A ranking of encodings.

The hierarchy

People judge position along a common scale most accurately, then length, then angle, then area, then colour intensity. Choosing a lower encoding costs precision.

That is why bars beat pies and why bubble sizes mislead.
Area errors are large: people underestimate how much a circle grew.

Preattentive attributes

Colour, size and orientation are noticed before conscious attention. Used sparingly they direct the eye; used everywhere they cancel out.