contour map

A whole family of a function's level curves, drawn together at evenly spaced output values.

A contour map is a whole family of level curves, drawn together at evenly spaced values of $k$ — the same idea a topographic map already uses, where each printed line marks one constant elevation. Where contour lines bunch tightly together, $f$ is changing rapidly over a short horizontal distance; where they spread apart, $f$ is changing slowly.

Examples

A weather map's isobars (constant-pressure lines) and isotherms (constant-temperature lines) are contour maps.