Contour line

The bottom part of the diagram shows some contour lines with a straight line running through the location of the maximum value. The curve at the top represents the values along that straight line.

A two-dimensional contour graph of the three-dimensional surface in the above picture.

Contour lines or isolines are used when plotting a function of 2 variables. All the points where the function has the same value are connected. Two well-known examples where such lines are common are: height lines drawn on topographical maps, and showing areas with the same pressure or temperature on weather charts. Contour lines are an application of level sets.

A map of Stowe VT with height lines
World map showing magnetic lines, for the year 2000
Weather diagram showing Meltemi winds over Greece and Turkey
Temperature diagram of Norway: areas with the same temperature range have the same colour.
The +10 degree celsius isotherm (July) is commonly used to define the Arctic region.

Contour Line Media

A three-dimensional surface, whose contour graph is below.
Edmond Halley's New and Correct Chart Shewing the Variations of the Compass (1701)
Isohyetal map of precipitation
Video loop of isallobars showing the motion of a cold front
From economics, an indifference map with three indifference curves shown. All points on a particular indifference curve have the same value of the utility function, whose values implicitly come out of the page in the unshown third dimension.