Equation for a Line The intercept--the point where the line crosses the vertical or Y-axis--is displayed in red. The intercept is the value of Y or of the equation when X = 0. The calculation of the slope--the change in Y when X changes by 1 unit--is indicated by the green lines and the value of the slope is displayed in green in the equation for the line. Moving the left and right scrollbars changes the intercept and slope, respectively.