 Mathematically, if we denote time by the variable formula_1, and speed by formula_2, then the function plotted in the graph would be denoted formula_3 indicating that formula_2 (the dependent variable) is a function of formula_1.Charts often include an overlaid mathematical function depicting the best-fit trend of the scattered data.This layer is referred to as a best-fit layer and the graph containing this layer is often referred to as a line graph.