The left half shows natural cubic splines, the right half shows the Hobby algorithm.
(Click on the links for videos about these topics.) Click
somewhere in the graph to add a point. You can also use the
mouse to move existing points. Press the Backspace key
to remove the last point and press
r to remove all points.
Press c to toggle between open and closed curves.
Press h to
toggle between showing both graphs and showing only the left
half. Press a digit to add a point on the graph after the n-th point. Press 0 to revoke the last digit.
Press w to switch between the "curl" values 0 and 1 for endpoints in open Hobby curves.