Using the subwindow 'C4' you can define the shape of a function by moving points with the mouse. The definition points are connected by a smooth spline curve:

The 'Points' submenu has commands to control the definition points: You can switch to a mode where a mouse click creates a new point, or one which let's you move the points, or a third one which deletes points by mouse click. With the option 'Continuous update' switched on you will have a live update of the curve while you move the definition points.

The values of the current curve of subwindow 'C4' can be entered in the main window's formula using the symbol C4.