Isolines Art Illustration: The U.S. Capitol
The United States Capitol is the seat of the United States Congress, the legislative branch of the U.S. federal government. It sits atop Capitol Hill, at the eastern end of the National Mall in Washington, D.C..
Wikipedia, United States Capitol.
A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value.
Isolines usually have a parallel trend.