isopleth

1. In common meteorological usage, a line of equal or constant value of a given quantity, with respect to either space or time;

same as isogram.

1. (Also called isarithm.) A line drawn through points on a graph at which a given quantity has the same numerical value (
   or occurs with the same frequency) as a function of the two coordinate variables.

   Compare isotimic line.

2. A straight line along which lie corresponding values of a dependent and independent variable.