Illustrates a basic idea about fitting a line to points: In a linear model, the effect of a change in X on Y is the same at all levels of X. The line has a constant slope of 0.1 in this example, so a difference of 100 in X makes a difference of 10 in Y, regardless of what X is.

The Move button makes the change in X and change in Y indicators move across the graph. Click again to stop the movement.
The Curve button shows a curved relationship between X and Y. Click it again to return to the linear relationship. In the curve model, a 100 difference in X makes a varying difference in Y. In this example, the Y difference gets smaller, then bigger, as X goes from 0 to 800.
The Line button switches you back to the line from the curve.