Contour line
A contour line (also isopleth, level set, isogram or isarithm) for a function of two variables is a curve connecting points where the function has a particular value. A contour map is a map showing contour lines.
The gradient of the function is always perpendicular to the contour lines. When the lines are close together the gradient is large: the variation is steep.
Different types of contour lines are given different names according to the nature of the quantity involved:
"Iso" can be replaced with "isallo" to give it the meaning of a line connecting points where some variable has changed at the same rate during some time period.
